[lipstick] Add a displayIsOn property to LipstickCompositor

I would have preferred to call it "displayOn", but there is a signal with that name already.
In the normal flow of events it /should/ not happen, but it is perfectly legal transition which can be made to happen -> IMO it is better to handle it correctly. The logic should be simpler and we avoid potential pitfalls [some vague memories of some old nokia device that failed to update display hw state when OFF->DIM occurred and WeirdStuff(tm) resulted].