Add docstrings for Library module
Fix unused variables in get_path_child_filenames

The 'value' variable was unused and is removed.
The 'meta' variable should have been used, but wasn't. It now is.
get_data_level not operating on SongRecord
Refactor get_crumb_data

This cleans up the call path slightly (with _crumb_data_by_parts),
and also removes the extraneous 'genre' part from the AlbumView
override.