Code review comment for lp://qastaging/~attente/unity-gtk-module/gtk-enable-mnemonics

I added the signal handler connection/disconnection in the unity_gtk_menu_shell_set_menu_shell () function because sometimes the GtkMenuShell can be set to NULL, and it makes sense to no longer watch once that happens. If you're worried about it being connected/disconnected more than once, in practice that function is never called with a non-NULL GtkMenuShell except at initialization.