[podcasts] Set media id when storing new podcasts. Minor code refactoring.
[podcasts] Set media id when storing new podcasts. Minor code refactoring.

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