Add two users of gtk_widget_insert_action_group

Add two users of gtk_widget_insert_action_group

Each GtkWindow with an associated GtkApplication should add this as
"app" to its action context.  Each GtkApplicationWindow is its own
GActionGroup, and it should add itself to itself with the prefix "win".

There is now some duplication here because we have the new GActionMuxer
hierarchy managed by GtkWidget, but GtkApplicationWindow still carries
its own muxer.  The redundancy will be removed in a future patch.