netwm: Un-tile before fullscreen

If a client switches to fullscreen, tiling would prevent the window from
being resized correctly, and the client window remains tiled even when
fullscreen.

Make sure to first un-tile the client window when transitioning to
fullscreen.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
Closes: xfce/xfwm4#478