Auto cleanup the dangling KWayland::Client::Surface returned from fromWindow

Clean up the Surface object returned from fromWindow if the internal
wl_surface object is destroyed. Otherwise it will stay there until the
QWindow is destroyed.

While we change the behavior of this object, it should generally be fine,
because the internal wl_surface pointer won't be valid and cause crash already
if the invalid wl_surface is referenced. Grepping through the existing code
base in KDE also seems to be ok to make this change.