QWaylandWindow should not inherit from the wl_surface wrapper

In the beginning, there was a direct mapping between a QWaylandWindow and a wl_surface.

With xdg_shell (and other shell) this is not the case anymore. Changing certain properties of a QWindow requires creating a new wl_surface (because it changes role). This is problematic because some parts of the platform plugin relies on this assumption to identify if the wl_surface has changed.

We should therefore use composition rather than inheritance. I.e. a QWaylandWindow may have a wl_surface.

Furthermore, ownership may not even be direct, i.e. the QWaylandWindow may have an xdg_surface which has a wl_surface.

This would also hopefully let us reuse the new wl_surface class for things that are not a QPlatformWindow, but still require a wl_surface, such as the cursor surface, or parts of window decorations.