Introduce workaround to make dvd playback work.

Currently the MediaElement has QUrl as source property,
which of course fails for anything but actual urls.
Four options present themself here:
a) use QVariant so that a QML consumer can throw a url at us
AND a mediasource
b) introduce MRL support
c) provide two interfaces, one for url and one for mediasource
d) functions on mediaelement to expose advanced features (e.g. payDvd)