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/folder.py

@@ -0,0 +1,9 @@
+from dataclasses import dataclass
+
+
+@dataclass(slots=True, frozen=True)
+class Folder:
+    name: str
+    path: str
+    is_sym: bool = False
+    trackcount: int = 0