Set proper return value from WM_DESTROY message on Windows (#75)

The documented return value for this message is an integer return code.

Author: cottsay

colcon_notification/desktop_notification/win32.py

@@ -124,3 +124,5 @@ def _on_destroy(hwnd, msg, wparam, lparam):  # noqa: D102
                 'Failed to delete the notification handle: {e}'
                 .format_map(locals()))
         win32gui.PostQuitMessage(0)
+
+        return 0