window: Workaround mismatch in GAction and GtkAction accelerators

... by removing the ones in GtkActionEntry's. As a consequence of this
the accelerators will not show up in the menu, but on the positive side
changing a shortcut will actually make the old one go away.

This should all be nice and dandy when use GMenu in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=728121