Some cleanup in GrlTrackerWrapper

This MR generalizes some duplicate code that I found while trying to figure out how gnome-music works:

• artist_filter_ids + artist_filter + artist_search_cb + self.props.source.query
• album_filter_ids + album_filter + albums_search_cb + self.props.source.query
• filter_ids + songs_filter + songs_search_cb + self.props.source.query

There are also some new docstrings.

The GrlTrackerWrapper.search function was very long, so I split it into multiple functions