Replace deprecated GtkAction / GtkActionGroup with XfceGtkActionEntry

Like done for thunar. Check xfce-gtk-extensions.h in libxfce4ui for details.

To be ckecked if the current accelerator management should be used, or if it makes sense to first switch to GAction, because the currently used GtkAccelMap will not be available any more in gtk4.

Hope I will have time to work on it sooner or later !