Allow subgroups in popup menues

Maybe related to

Improve the way how very long popup menu lists are handled. I have a couple of popup menu lists which are longer than my screen height, 1920 pixels, and selecting an element which is offscreen is quite tedious.

Maybe PopupMenu can be extended to allow "subgroups".
The "Procedures" popupmenu in IP7 does that already for menu entries vs functions.