modularize src

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

swingmusic/plugins/register.py

@@ -0,0 +1,18 @@
+from swingmusic.db.userdata import PluginTable
+from sqlalchemy.exc import IntegrityError
+
+
+def register_plugins():
+    try:
+        PluginTable.insert_one(
+            {
+                "name": "lyrics_finder",
+                "active": False,
+                "settings": {"auto_download": False},
+                "extra": {
+                    "description": "Find lyrics from the internet",
+                },
+            }
+        )
+    except IntegrityError:
+        pass