Fix deep diff comparison of GObjects

This is a fix for issue #340 (closed). After some investigation, it turns out that the deep diff functionality is not working properly with classes that derive from GObject. This is because at some point the props attribute is inspected by deepdiff and it tries to get the class using class. However, the attribute props contains a list of GObject properties that don't contain the attribute class. Whether that's a bug in Gtk or intentionally is unclear to me. The fix is to ignore the comparison of the props attribute. Note that the object to compare already contains named attributes with the values to compare, so comparing also .props is kind of redundant. Although the issue #340 (closed) referred to playlist the issue was actually also present in the artist and browse views.