Test whether the C++ compiler works by compiling a test program

Show some hidden VTE preferences in the Various preferences page

Follow-up to #1168.
Closes #1170.

This is more reliable than using `which`, which doesn't work if the CXX
environment variable is set to something else than an executable (e.g
contains options), and is apparently less portable on some systems.

See:
 * http://lists.geany.org/pipermail/devel/2009-September/001367.html,
   which lead to 5bb28825.
 * https://sourceforge.net/p/geany/bugs/455/, which lead to a revert
   of the above, 5b9605a9.

Fixes #829.

Move TM and ctags files

Minor README updates

At the moment tags with identical names are stored into a linked list in
tags_table and parents_table. This however leads to quadratic complexity
when looking up the nearest parent or tag in tree because the whole list
has to be traversed.

Use binary trees indexed by line number instead of lists so the lookup can
be performed in log(N) time and the overall complexity is N*log(N) instead
of N^2.

The GTree API is a little stupid because during the search it doesn't give
access to the value and it doesn't tell when a leaf node was reached. For
this reason the lookup has to be made in two steps - first, the best line
number is found (returned in user_data) and then a normal search for the
found line number is made to get the value stored in the tree.

This patch fixes the problem described in #577 when e.g. a big json export
file contains many identically named tags.

No more needed using MSYS2.

The version of MIO corresponds to commit 509a47dbc in universal-ctags
which contains just minimal changes mostly related to changing MIO from
a library into a single ctags source file:

- replaced the glib types with ordinary C types
- removed the "virtual" calls and replaced them with simple if/else
- made the implementation in a single file
- reformatted the library to more or less match universal-ctags style
- removed the MIO_FORCE_ANSI ifdef as it included some glib file and
  we don't really need it
- added mio_flush() - of course makes sense just for the file backend
  (calls fflush())
- made mio_free() return error code from fclose()
- changed mio_new_fp() to return NULL when the passed FILE is NULL
  (simplifies logic at one place in ctags and makes sense IMO)

Move the tag manager implementation to src as it really is part of Geany
sources and start making the ctags directory structure similar to
the universal-ctags one.

In principle, the patch does

mv tagmanager/src src/tagmanager
mv tagmanager/ctags tagmanager/main
mv tagmanager ctags

plus corresponding Makefile.am and configure.ac updates.

Custom theming fixes for Adwaita 3.20/GTK 3.20

Follows the GTK3 behavior, and avoid text becoming invisible if the
selection background is too light (unlikely, but possible).

Adwaita 3.20 on GTK2 uses the pixmap engine to set a background image
on all states of all GtkEntries.  Earlier versions did the same but
with a transparent background, thus not hiding our background color.

Fixes #1135, fixes #1101.

This fixes the plugin ABI break that happened in previous commmit.

Closes #1134

- slight rst formatting improvements
- Mac OSX -> Mac OS
- punctuation

Say hello to Geany 1.29 "Jowar"!

Windows installer: C and GTK runtime installation

Don't require plugin key group name and label strings to be static

Take a copy of the strings not to require them to live live as long as
the plugin does.

This is mostly useful for plugins implemented in dynamic languages
(e.g. through a plugin proxy), as most C plugins will use a static
string here; but it makes the API more straightforward and avoids odd
issues if any plugin doesn't use static strings here, even C ones.

Closes #1125.