modularize src

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

swingmusic/utils/wintools.py

@@ -0,0 +1,18 @@
+import platform
+
+IS_WIN = platform.system() == "Windows"
+
+
+# TODO: Check is_windows on app start in settings.py
+def is_windows():
+    """
+    Returns True if the OS is Windows.
+    """
+    return IS_WIN
+
+
+def win_replace_slash(path: str):
+    if is_windows():
+        return path.replace("\\", "/").replace("//", "/")
+
+    return path