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