Add Qt::CustomWindow window type

The custom window type can be used to indicate a window which can be
set to some custom platform specific window type using the platform's
native interface.

This will allow for example to get a plain wl_surface instead of a
wl_shell_surface assigned to a QWindow in QtWayland.

Change-Id: Ib0699955367b467cc5eb5ecfa1942db5e8076348