QVideoWidget had to be abandoned in favour of QGraphicsVideoItem. This
is because UBGraphicsMediaItem, i.e the class representing a media
(audio or video) object on the board, is a QGraphicsProxyWidget, and is
used to embed a QWidget into the Scene.

With Phonon's video widget, it was possible to embed the video widget in
this ProxyWidget. This is no longer possible (except on Windows, for
some reason), so this commit is a workaround, to use a
QGraphicsVideoItem instead of a QVideoWidget while modifying the rest
of the class hierarchy as little as possible.

Ultimately, a cleaner solution (not making UBGraphicsMediaItem inherit
QGraphicsProxyWidget, for example) may be desirable.