refactor: move music db into a separated file.

IMPORTANT NOTE:
The default database path has changed. It was database.db, but
now they ARE settings.db and music.db.

Database migration will detect music table in settings.db, and
move it out of settings.db.

To update from old version, you need to
 1. if you use your own db path settings in the command
    line or in configuration.ini, you don't have to change
    anything;
 1. or,if you use default database path, rename database.db
    into settings.db.

mumbleBot.py

@@ -23,7 +23,7 @@ from packaging import version
 import util
 import command
 import constants
-from database import SettingsDatabase, MusicDatabase
+from database import SettingsDatabase, MusicDatabase, DatabaseMigration
 import media.system
 from media.item import ValidationFailedError, PreparationFailedError
 from media.playlist import BasePlaylist
@@ -666,7 +666,9 @@ if __name__ == '__main__':
     parser.add_argument("--config", dest='config', type=str, default='configuration.ini',
                         help='Load configuration from this file. Default: configuration.ini')
     parser.add_argument("--db", dest='db', type=str,
-                        default=None, help='database file. Default: database.db')
+                        default=None, help='settings database file. Default: settings.db')
+    parser.add_argument("--music-db", dest='music_db', type=str,
+                        default=None, help='music library database file. Default: music.db')
 
     parser.add_argument("-q", "--quiet", dest="quiet",
                         action="store_true", help="Only Error logs")
@@ -691,20 +693,21 @@ if __name__ == '__main__':
 
     args = parser.parse_args()
 
+    # ======================
+    #     Load Config
+    # ======================
+
     config = configparser.ConfigParser(interpolation=None, allow_no_value=True)
+    var.config = config
     parsed_configs = config.read([util.solve_filepath('configuration.default.ini'), util.solve_filepath(args.config)],
                                  encoding='utf-8')
-    var.dbfile = args.db if args.db is not None else util.solve_filepath(
-        config.get("bot", "database_path", fallback="database.db"))
-
     if len(parsed_configs) == 0:
         logging.error('Could not read configuration from file \"{}\"'.format(args.config))
         sys.exit()
 
-    var.config = config
-    var.db = SettingsDatabase(var.dbfile)
-
-    # Setup logger
+    # ======================
+    #     Setup Logger
+    # ======================
 
     bot_logger = logging.getLogger("bot")
     formatter = logging.Formatter('[%(asctime)s %(levelname)s %(threadName)s] %(message)s', "%b %d %H:%M:%S")
@@ -732,14 +735,32 @@ if __name__ == '__main__':
     logging.getLogger("root").addHandler(handler)
     var.bot_logger = bot_logger
 
+
+    # ======================
+    #     Load Database
+    # ======================
+    var.settings_db_path = args.db if args.db is not None else util.solve_filepath(
+        config.get("bot", "database_path", fallback="settings.db"))
+    var.music_db_path = args.music_db if args.music_db is not None else util.solve_filepath(
+        config.get("bot", "music_database_path", fallback="music.db"))
+
+    var.db = SettingsDatabase(var.settings_db_path)
+
     if var.config.get("bot", "save_music_library", fallback=True):
-        var.music_db = MusicDatabase(var.dbfile)
+        var.music_db = MusicDatabase(var.music_db_path)
     else:
         var.music_db = MusicDatabase(":memory:")
 
+    DatabaseMigration(var.db, var.music_db).migrate()
+
     var.cache = MusicCache(var.music_db)
 
-    # load playback mode
+    if var.config.get("bot", "refresh_cache_on_startup", fallback=True):
+        var.cache.build_dir_cache()
+
+    # ======================
+    #   Load playback mode
+    # ======================
     playback_mode = None
     if var.db.has_option("playlist", "playback_mode"):
         playback_mode = var.db.get('playlist', 'playback_mode')
@@ -751,12 +772,13 @@ if __name__ == '__main__':
     else:
         raise KeyError("Unknown playback mode '%s'" % playback_mode)
 
+    # ======================
+    #  Create bot instance
+    # ======================
+
     var.bot = MumbleBot(args)
     command.register_all_commands(var.bot)
 
-    if var.config.get("bot", "refresh_cache_on_startup", fallback=True):
-        var.cache.build_dir_cache()
-
     # load playlist
     if var.config.getboolean('bot', 'save_playlist', fallback=True):
         var.bot_logger.info("bot: load playlist from previous session")