Allow resetting Qt::WindowFullscreenButtonHint

Currently Qt::WindowFulscreenButtonHint is only
respected on window creation. But flags
can also be adjusted later on.
Further setWindowShadow can be removed from
within createNSWindow, as it operates on a Nil
object in that case. It is however called by
recreateWindow / setWindowFlags subsequently.

Change-Id: I507d6fde5ad2f0ee5b9db322325ede99b70e151e
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>