There are cases where plugin functionality requires changing the behavior of the WindowProc of the top-level HWND and/or the Flutter HWND. Examples:

• [window_size] Allow setting min/max sizes google/flutter-desktop-embedding#483
• Windows HSCURSOR not staying updated google/flutter-desktop-embedding#636

We should allow plugins to register as delegates of the top-level WindowProc (if the app chooses to provide it; for add-to-app this would not always make sense, so we should be clear that it is not required to work) and the Flutter WindowProc (if we keep an HWND-based approach at that level).