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@

multimedia/openshot/Makefile

@@ -1,6 +1,7 @@
 PORTNAME=	openshot
 DISTVERSIONPREFIX=	v
 DISTVERSION=	3.1.1
+PORTREVISION=	1
 CATEGORIES=	multimedia
 
 MAINTAINER=	tatsuki_makino@hotmail.com
@@ -21,7 +22,7 @@ USES=		desktop-file-utils pyqt:5 python shared-mime-info shebangfix
 USE_GITHUB=	yes
 GH_ACCOUNT=	OpenShot
 GH_PROJECT=	${PORTNAME}-qt
-USE_PYQT=	pyqt5
+USE_PYQT=	pyqt5 webengine
 USE_PYTHON=	distutils optsuffix
 SHEBANG_FILES=	src/language/generate_translations.py\
 		src/language/show_translations.py\