gmenu: Move the main menu into the .glade file.

* glom/appwindow.[h|cc]: Move the GMenu XML into:
* ui/operator/window_main.glade
  and use it from the regular Gtk::Builder instance.

The GMenu XML needs to be in a separate file so that
its human-readable parts can be translated.

However, glade does not seem to support GMenu yet,
so we cannot edit this file inside glade.