How Screens Get Managed
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* PlasmaApp handles the views
* DesktopCorona handles the containments

So:
* When a screen is added: DesktopCorona adds a new containment if needed, since it handles Containments and the 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
