Add a Qt5 option to multimedia/mlt

multimedia/mlt has a Qt4 option specifically to support multimedia/kdenlive.
Future versions of kdenlive are Qt5 based. Prepare for this by also enabling
the Qt5 library being built.

Convert the QT4 option into a radio-option "QT=QT4 QT5".

Reviewed by: mat
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D7432