Introduce SurfaceCursorSource

The contents of the cursor can be either a normal image or a wl_surface.
At the moment, we get an image from the wl_surface. But it's going to be
changed so we use SurfaceItem to render wl_surface cursors.

SurfaceCursorSource provides the way to get the wl_surface handle from
the cursor. In order to provide backwards compatibility, it also provides
a QImage.