[podcasts] Refactor code so that media creation is done only in a single place.
[podcasts] Refactor code so that media creation is done only in
a single place.

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