gladeui/glade-app.c: Load composite templates from G_USER_DIRECTORY_TEMPLATES

gladeui/glade-project.[ch]
 o Added glade_project_dump_string()
 o Addes safe guards for NULL catalogs (composite template adaptors does not have a catalog)

gladeui/glade-widget-adaptor.[ch]
 o Added template and template-path properties
 o Added glade_widget_adaptor_get_template() and
   glade_widget_adaptor_from_composite_template()

gladeui/glade-composite-template.[ch]: Added support to load composite templates and export a widget as such.

plugins/gtk+/glade-gtk.c, plugins/gtk+/gtk+.xml.in: added "Export as template" action