Augment testing for Glade2 UI files.
Support for GtkBuilder UI files.

Fix Savannah bugs #29216, #29900 and #34506.
Tests for GtkBuilder UI files.
Document GtkBuilder UI files support.
Extract libexpat compatibility layer to libexpat-compat.{c,h} files.
Support for GSettings schema files.
Splitted gtkbuilder patch.