gdk: Deprecate compositing support

We might want to remove support for it completely and always return
FALSE.
testgtk: Don't trigger deprecation warnings

.. and various other assorted fixes, mostly related to drawing code that
assumed stuff wasn't transparent by default.
tests: Update testoverlaystyleclass test

Make it not use deprecations anymore and use CSS to show things work by
changing border-radius when a style class exists.
tests: Don't force black background in testwindows
tests: Update testtooltips for deprecations
tests: Update testexpand example for deprecations
examples: Add deprecation guards

Apparently it's not possible in GtkTextView to say "this style should
apply to all text, including newly inserted text. Or I'm just too stupid
to use its API.

So instead, keep using override_color().