Load the file chooser's widgets from the .ui file

Load the file chooser's widgets from the .ui file

This is Tristan's *excellent* work, minus the old code for the shortcuts bar - that is all done
in GtkPlacesSidebar now.

The UI gets loaded from a Glade resource; most of the old code to create the UI by hand is gone.

There is still code for save_widgets_create(); this needs to be moved into the UI file, but it
is not a big deal.

gtk_file_chooser_default_init() calls a new post_process_ui() that takes care of all the things
that cannot be done directly in Glade.