Add support for GTK frame extents

This is required for proper positionning and
resizing of CSD windows from GTK3/GNOME.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
Add new default themes

To better integrate with GTK3/GNOME apps.
Also include hdpi and xhdpi variants.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>