tracker: Ensure that Video is preferred to Image
tracker: Ensure that Video is preferred to Image

When creating a new object, prefer creating a Video to creating
an Image when the object has both RDF types.

https://bugzilla.gnome.org/show_bug.cgi?id=730028

jasuarez/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,GNOME/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,jasuarez/grilo-plugins,MathieuDuponchelle/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins,grilofw/grilo-plugins