track thumbnail from the first track in an album that has one.
+ rewrite Populate.remove_modified with sets
+ clean the SqliteManager utility class
+ Rewrite ProcessTrackThumbnails to use a process pool instead of a thread pool
+ rewrite track store's remove_tracks_by_filepaths to utilize sets
+ use getters instead of constants in settings classes
+ refactor previous references
+ move get_xdg_config_dir() from settings.py to app.utils.xdg_utils.py
