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/plugins.py

@@ -0,0 +1,10 @@
+from dataclasses import dataclass
+
+
+@dataclass
+class Plugin:
+    name: str
+    active: bool
+    settings: dict
+    extra: dict
+