Second part of #1047.

Part of #1047.

And explicitly convert backward slashes in the global
prefix to forward slashes as tools like pkg-config or
gcc could interpret the backward slashes as escape
sequences.

Since the infobar is backed in a GtkPaned that allows "shrink", if the
content is too wide, it's far end will just get cropped.  Also, wrap
labels are nicer to display possibly long texts as it flows naturally.

Now document_get_from_page() supports arbitrary nesting inside the
notebook page, update document_get_notebook_page() to support it also,
by searching up for the direct notebook child rather than assuming it
is the ScintillaWidget's direct parent.

Cleanup some useless placeholders, and replace some one-column tables
with vertical boxes.

No UI changes.

This avoids glade from emitting some useless placeholders.
No UI changes.

Directly use the GeanyDocument rather than pass the notebook page
widget and get the document from that.  This makes the code more future
proof and less weird.

Since infobars notebook_tab_close_clicked_cb() wants a GeanyDocument as userdata.
show_tab_bar_popup_menu() on the other hand wants the actual GtkNotebook child.

This is an extension of the long-lived `document-messages` branch.

It is mostly adjusted according to the feedback on the mailing list.
There might existing some usability issues to be resolved, but this
is the first step towards non-modal document notifications.

infobars: Intercept some keys when the info bar is shown to allow interacting with it using the keyboard only.

When the info bar is shown tab/shift+tab and escape are intercepted.
* tab/shift+tab change the focus to the info bar buttons which can then be
  cycled through with more tab presses
* escape closes the info bar (same as clicking cancel)

Both keys aren't needed for the document because it's read-only. Other keys,
such arrow/page up/down, remain to the document for navigating.

When the file was deleted from disk the message that the file is newer on
disk has become out of date and irrelevent.

last one (respectively) is significant to the user.

This avoids accidental changes the file until the infobar is ackowledged. The
document can still be viewed and scrolled through but modifications and saving
are disabled. Of course ignoring the document by changing to another one is
also possible.

Supports changes to the widget heirarchy in the future.

The close button is removed from the dialog also since the user can easily
choose to close the document normally.

Setting default response removed since it's not working yet.

Two other related changes are that document_set_text_changed() is used to
indicate that the buffer is different from what's on disk and also that
the 'Close' button has been removed from the dialog since the user can easily
just close the document themselves the normal way.

This is to allow stacking widgets such as GtkInfoBar above the Scintilla
widget in each tab.

notebook.c need to be changed because the document isn't the direct widget
anymore which was assumed for tab closing.

Re-order win32.[ch] includes a little bit to unbreak build

No code changes.

Closes #272

* If GETTEXT_PACKAGE is not defined (ex. by build system/user) then
  define it to NULL. Most GLib gettext-functions allow passing NULL
  to use the domain set with textdomain().
* Improve the ngettext() wrapper macro.
* Improve/fix bind_textdomain_codeset() wrapper macro.
* Fix argument in bindtextdomain()
* Cast bind*textdomain*() results with (void) to get rid of compiler
  warnings when building without GETTEXT_PACKAGE defined.

This is a mega-commit - because most of it had to be done in one go
otherwise some commits would fail to compile - that attempts to fix a
few problems with Geany's includes as well as various other related
cleanups. After this change it's easier to use includes and there's
little worry about which order things are included in or who includes
what.

Overview of changes:

* Include config.h at the start of each source file if HAVE_CONFIG_H
  is defined (and never in headers).
* Go through each source file and make the includes section generally
  like this:
  - Always config.h first as above
  - Then if the file has a header with the same name, include that
  - Then include in alphabetical order each other internal/geany header.
  - Then include standard headers
  - Then include non-standard system headers
  - Then include GLib/GTK+ related stuff
* Doing as above makes it easier to find implicit header include
  dependencies and it exposed quite a few weird problems with includes
  or forward declarations, fix those.
* Make geany.h contain not much besides some defines.
  - Add a little header file "app.h" for GeanyApp and move it there
  - Move "app" global to new "app.h" file
  - Move "ignore_callback" global to "callbacks.h"
  - Move "geany_object" global to "geanyobject.h"
* Add an include in "geany.h" for "app.h" since GeanyApp used to be
  defined there and some plugins included this header to access
  GeanyApp.
* Include "gtkcompat.h" everywhere instead of gtk/gtk.h so that
  everywhere sees the same definitions (not a problem in practice AFAIK
  so this could be changed back if better that way.
* Remove forward declarations from previous commits as some people
  apparently consider this bad style, despite that it reduces inter-
  header dependencies.

TODO:
* As always, to test on win32
* As always, to test with not Autotools
* Test plugins better, both builtin and geany-plugins, likely API/ABI bump
* Test with various defines/flags that may change what is included
* win32.[ch] not really touched since I couldn't test

* Always define GEANY_FOO_H to 1 in the header guards
* Always put a G_BEGIN_DECLS/G_END_DECLS guard in every header for
consistency, even private ones where it doesn't matter.
* Always include either <glib.h>, <gtk/gtk.h> or some other header
that will provide G_BEGIN_DECLS before using it. In a lot of headers
that use glib.h and gtk/gtk.h stuff anyway, this resolves an implicit
dependency they had on them being included before that header.
* Always put a comment at the #endif part of the guard so it's
easier to see what it applies to.
* Always use an underscore between the header guard identifier's words
even though the filename doesn't have one.

This helps to avoid the requirement that geany.h be included before
using any of the changed headers.

This is for work on making the files scannable by GObject-Introspection
but is still useful otherwise (even fixes a FIXME in the comments). I
made this by using a simple GNU Make file and trying to compile the
sources each on their own without all the build system infrastructure.

* Add keybindingsprivate.h file to hold private GeanyKeyGroup structure
and remove it from the GEANY_PRIVATE guard in keybindings.h.
* Move private members that were guarded by GEANY_PRIVATE from
GeanyFiletypes to GeanyFiletypesPrivate and remove guarded build.h
include.
* Move private members that were guarded by GEANY_PRIVATE from
GeanyProject to GeanyProjectPrivate.