Support for cascading custom actions (#221)

Actually, I mostly just did it the same way as in thunar:

https://gitlab.xfce.org/xfce/thunar/-/blob/master/thunar/thunar-action-manager.c#L2307

https://gitlab.xfce.org/xfce/thunar/-/blob/master/thunar/thunar-gtk-extensions.c#L80

Not sure if it was a good idea to use tooltips ... suppose I should remove them, since the other items as well seem to have no tooltips.

Closes #221 (closed)