Prefer the GDK windowing backend for Clutter

If Clutter has a GDK backend compiled in, we want to use that to
minimize the interaction surface between GTK and Clutter.
build: Depend on Clutter ≥ 1.18
actor: Ignore deprecated API

We really need to drop the internal usage of ClutterX11TexturePixmap.
embed: Ignore deprecation warnings

We *really* need to disable double buffering in GTK, otherwise we
flicker horribly.
offscreen: Fix deprecation warnings

Don't use deprecated API, or disable deprecation warnings if that's not
possible without a more thorough investigation.
texture: Ignore deprecation warnings

The whole GtkClutterTexture class should be deprecated, and replaced by
a GtkClutterPixbuf class that implements the ClutterContent interface.
util: Ignore deprecation warnings
window: Drop deprecated API
build: Drop INCLUDES directives

They have been long since deprecated by AM_CPPFLAGS and AM_CFLAGS.
examples: Remove unused variables

Silences the compiler warning.
Release Clutter-GTK 1.5.4
Post-release version bump to 1.5.5