This should avoid confusion in the toolbar customize dialog to not have
two identical named items.

* Add new function: document_update_tags().
* Refactor the various tag update functions into document_update_tags().
* Remove extra call to update the tags in document_new_file().

Found by codebrainz using clang static analyser.

This prevents losing the label edit input dialog below the build command
dialog which can be confusing since it prevents the build command dialog
from accepting input even though it's on-top of the (potentially hidden)
label edit input dialog.

Uninitialised GValue does not always work, but is an opaque type so
structure of initialiser isn't known.  Glib 2.30 on has G_VALUE_INIT
to use as initial value.  Fix so if not defined give it the previous literal
value { 0 }, although this leaves the warning, so it is not initialising
the GValue correctly but enough to work.

Fix warnings that appeared with GCC 4.6
js.c:1067:10: warning: variable ‘is_prototype’ set but not used
dialogs.c:173:2: warning: missing initializer
dialogs.c:173:2: warning: (near initialization for ‘value.data’)
sidebar.c:534:17: warning: unused variable ‘doc’

Closes #3432760.

Not every filetype for a specific lexer implements type keywords even if the
lexer supports it.

This prevents GTK of trying to fetch the necessary information like
MIME-type itself, which leads to a significant speedup (> 30%), as
well as using the real MIME-type we use rather than the GIO-guessed
one.

Use foreach_document() in a few places where appropriate and make
some code more consistent.

These add noise to the code with little real benefit.

This function was only used from document_create() and most of this code is
not needed since the memset() and g_new0() calls set the memory to all 0's,
which in this case should suffice to (re)set all the members to 0/NULL/FALSE.

Refactor so all the resetting to defaults code is done in remove_page()
only and then only do the required non-FALSE/NULL initialization in
document_create().

Move the remove_page() prototype to the top of the file and various other
minor changes in remove_page().

It was used only in one place in document_update_type_keywords() which
already did a similar check using the file type before calling this function.

Update HACKING file and very minor cleanup of other code in
document_update_type_keywords().

Previously an error message was shown if doc->file_name is NULL.

The Save As dialog is now shown if the document does not have an
absolute path. This is because the user should confirm where to save
the document in this case.

Although this changes plugin API behaviour, it seems the best way to
ensure the Save As dialog is always shown when needed so the user
knows where the document has been saved.

Also fixes a doxygen warning.

Fix correctly checking result of editor_lexer_get_type_keyword_idx().
Do not check IS_SCINTILLA unnecessarily (this is covered by DOC_VALID).
Do not use G_[UN]LIKELY macros as this complicates code without any
real benefit (unless in a tight loop).

Unix path separators can occur e.g. when using MSYS.
Windows does not allow filenames to contain Unix path separators so
this should be safe.

Finding paths in the tree was not case-insensitive either.

Use case-insensitive matching for project base path and home
directory in document filenames.

This is useful if one have many open files from different directories
and wants to see the files from only one or another.

This makes the code more readable, potentially more future-proof (if
the actual string changes) and better style (catches possible typos at
build-time).