This commit implements a vertical menu widget, which should be quite a bit more flexible than
what we currently have.
It defines interfaces in order to respond to selection changes, show and hide. And has a i_item_widget
interface class to allow you to use custom item widgets.
This is done in preparation for adding a debug menu, in which I kinda want to add toggle options
while using the same vertical_menu widget.
Right now, vertical_menu is only used in Select_Menu. Needless to say that Select_Menu was reworked quite a bit.
Still, in terms of visuals or functionality, the changes should be invisible for now. I mean, I didn't do anything *new* with it yet.
