Qt Wayland Client public API

The Qt Wayland Client module would benefit from more APIs: Each of the individual shells have special things you may want to set (some of which are currently exposed through environment variables because we don't have anything better).

In addition, there are handles you would want to access for manipulating surfaces and other objects on the connection. On most platforms, a window will be represented by a single handle that we expose through winId(), but since it is not clear which object to expose through this on Wayland, we currently just return a serial number there.