add --config flag to modify config path

+ 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

app/migrations/__preinit/move_to_xdg_folder.py

@@ -17,7 +17,7 @@ class MoveToXdgFolder:
     @staticmethod
     def migrate():
         old_config_dir = os.path.join(Paths.USER_HOME_DIR, ".swing")
-        new_config_dir = Paths.APP_DIR
+        new_config_dir = Paths.get_app_dir()
 
         if not os.path.exists(old_config_dir):
             log.info("No old config folder found. Skipping migration.")