Support for GtkBuilder UI files.

Fix Savannah bugs #29216 and #29900.
Add an ifndef guard to x-glade.h.
Tests for GtkBuilder UI files.
Document GtkBuilder UI files support.
Extract libexpat compatibility layer to libexpat-compat.{c,h} files.

It will be used at GSettings extractor.
Add one-dot-more-extension search for .xml files.
Support for GSettings schema files.