python: in test-ui, convert to strings when storing media info
python: in test-ui, convert to strings when storing media info

Since the TreeModel stores strings, we need to get sure that
the data types being inserted are the right ones.

MathieuDuponchelle/grilo,jasuarez/grilo,jasuarez/grilo,GNOME/grilo,kyoushuu/grilo,grilofw/grilo,MathieuDuponchelle/grilo,grilofw/grilo,kyoushuu/grilo,jasuarez/grilo,kyoushuu/grilo,MathieuDuponchelle/grilo,kyoushuu/grilo,grilofw/grilo,GNOME/grilo