multimedia/openshot: Prepare for www/qt5-webkit removal

www/qt5-webkit is scheduled to be removed at the end of the year [1]
along with its corresponding python bindings from devel/py-qt5-pyqt. [2]
In the meantime, this port will continue to default to the WebKit
backend unless otherwise specified via the commandline arguments
'-b webengine' or, synonymously, '--web-backend webengine'. After
www/qt5-webkit deletion, it will automatically use the bindings from
www/py-qt5-webengine.

[1] https://cgit.freebsd.org/ports/commit/?id=e8e65e7574de0b374caaa3da0ba6a26a67103e62

PR: 273866 [2]
With hat: kde@