Add TODO comments to derive from Actionable when we can break ABI.

* gtk/src/button.hg:
* gtk/src/menuitem.hg:
* gtk/src/switch.hg:
* gtk/src/toolbutton.hg: These classes need to derive from
  Gtk::Actionable so we can, for instance, call
  set_action_name() on them to use them with a Gio::Action that
  is in an action group.
  However, we cannot do this without breaking ABI.