Complete rebuild of status screen system
The status screen has been moved from "screens/overlays/" to "ui" The
various names floating about (overlay, subscreen, nested screen etc)
have been consolidated into the following:
- StackableScreen - An interactive screen that can have other screens
stacked on top of it but will keep its original state. These
screens usually contain a menu and a number of panels.
- SubscreenPanel - A panel that is part of a StackableScreen. They can
gain and lose focus, as well as push screens onto the stack.
StackableScreens are aware of transitions, so screens don't transition
in until the previous screen has transitioned out.
(of course, all this might change again...)
