Although GtkPlacesSidebar is a public widget, we can't have that in Glade's general catalog
as this branch is not merged into GTK+ proper yet. We'll move the widget to Glade's
catalog when this is done.
This catalog can be used to work with GTK+'s private widget types,
this patch exposes a private function gtk_glade_catalog_init() which
Glade will use for the sole purpose of initializing some private widget
types in GTK+ that are referenced from various GTK+ composite widget
xml files.