Different behavior in single- vs. multi-process mode

One would expect that appman behavior is the same when running in single- or multi-process mode. This is to keep track of some deviations:

1. In MP an ApplicationManagerWindow's color is interpreted as pre-multiplied alpha, whereas in SP it's straight alpha. This is due to the fact, that a regular Window expects pre-multiplied in contrast to a Rectangle type that expects straight alpha. One could adapt one or the other with a favor to use straight alpha for both. However, that won't be backwards compatible. An alternative would be to introduce an extra property like "backgroundColor".
2. An ApplicationManagerWindow's opacity property cannot be set, although in MP ApplicationManagerWindow is a QWindow which has an opacity property. In SP one might consider deriving InProcessSurfaceItem from a QQuickRectangle instead of a QQuickItem to support opacity and other properties.
3. Setting the input focus to a WindowItem propagates the key events to the appropriate application in SP, whereas in MP the application only gets focus when it is clicked. In MP a WindowItem should forward the focus to it's QWaylandQuickItem delegate (through the WaylandImpl) to make the application get input events.