How Screens Get Managed
-----------------------
* PlasmaApp handles the views
* DesktopCorona handles the containments with the help of Activity classes

So:
* When a screen is added: DesktopCorona tells the current Activity, which adds a new containment if needed. PlasmaApp responds to their creation
* When a screen is removed: PlasmaApp removes the associated view(s); we don't want the Containment to go away, however, since the screen may return
* When a screen changes geometry: the Views (DesktopView, PanelView) change their geometry and that of their containments if needed

Future work:
* Migration of panels from non-existent screens to existing screens (perhaps asking if the user would like them to migrate?)
* 'hibernation' of containments from non-existent screens (Activity will handle this)