modularize src

+ merge main.py and manage.py
+ move start logic to swingmusic/__main__.py
+ add a run.py on the project root

swingmusic/models/__init__.py

@@ -0,0 +1,16 @@
+from .album import Album
+from .track import Track
+from .artist import Artist, ArtistMinimal
+from .enums import FavType
+from .playlist import Playlist
+from .folder import Folder
+
+__all__ = [
+    "Album",
+    "Track",
+    "Artist",
+    "ArtistMinimal",
+    "Playlist",
+    "Folder",
+    "FavType",
+]