This lets the View have a desktop set to it and when drawing a Containment we pass in the effective desktop # being currently painted. The Containment is free to ignore this information, or it can try and cast the passed in options to a Containment::StyleOption to retrieve this information.

This should be used for any per-desktop painting that happens (e.g. separate background per desktop) and opens the way to having View-per-Desktop for better composite manager support (think: desktop grid or cube where more than one desktop is simultaneously visible)