GtkWindow: Avoid an early exit

When gtk_window_set_titlebar (win, NULL) is called, we were taking
an early exit and forgot to re-map the window. This does not normally
happen in practice, but glade is about to get a 'csd' switch which
lets one toggle back and forth between titlebar and no titlebar.
Adwaita: dimmed backdrop foreground color

To make focused window more evident.
Adwaita: use $backdrop_fg_color where it should

.background:backdrop was not using that one so some labels didn't
look right in backdrop.
Adwaita: darkened a tiny bit bright variant background

Trying to differentiate a bit more the focused window from the
backdrop ones.
gtkgesturesingle: Unset button/sequence if parent handle_event failed

There are legit reasons for GtkGesture::handle_event to return FALSE,
GtkGestureSingle objects should be unsetting the current button/sequence
if that happens, in order to avoid inconsistent states.

https://bugzilla.gnome.org/show_bug.cgi?id=738591
Updated Kazakh translation
Adwaita: comment exported colors
Adwaita: add back text color.

See https://bugzilla.gnome.org/show_bug.cgi?id=726316 for details.
Adwaita: darken a tiny bit more bright variant background color
Some documentation updates

Mention libepoxy in various places as a new dependency, and also
mention Wayland in the build documentation.
Work around a pkg-config bug

It doesn't report -I${prefix}/include in cflags, even if .pc
files explicitly put it there. This was breaking the build
outside of a jhbuild shell when libepoxy is in the jhbuild tree
but not in /usr.
GtkWindow: Add a getter for the custom titlebar

This is needed for the glade integration of CSD windows.
Fix a few typos
Fix the build
docs: documentation for GtkTextBufferClass

It'll most probably be more useful in the near future.
docs: add missing symbol in GtkTextView