Add more API to change QWaylandSurface properties

The previous patch allows for QWaylandSurface to call into custom
protocol implementations, but the other way around is needed too.
Instead of adding many public setters to QWaylandSurface this
makes QWaylandSurfaceInterface the way to set them, through
protected functions. This avoids calling the setters from other code
paths, which would bring the server and client states of the same
surface out of sync.