modularize src

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

run.py

@@ -0,0 +1,9 @@
+# Launcher script
+
+import multiprocessing
+import swingmusic.__main__ as app
+
+if __name__ == "__main__":
+    multiprocessing.freeze_support()
+    multiprocessing.set_start_method("spawn")
+    app.run()