Keyboard input doesn't work

When the System UI sets the keyboard focus to a WindowItem, key events won't get to the actual application represented by this WindowItem. The reason is that a WindowItem is the parent of the actual application's (surface) item (QWaylandQuickItem in MP and InProcessSurfaceItem in SP) and key events are, of course not forwarded to children. Hence, the focus or key events should explicitly be forwarded from WindowItem to the application's (surface) item.