Add support for transparency effects (and wallpaper tinting) on macOS.

[Adrian-Samoticha]

Hello,

I am the maintainer of the macOS port of flutter_acrylic and the author of the macos_window_utils package which it depends on.

In a discussion on the macos_ui repository we've been discussing if it was possible to apply the transparency effects in flutter_acrylic/macos_window_utils to windows created with desktop_multi_window. As far as I can tell, it should be possible (at least on macOS), however, changes to desktop_multi_window would need to be made.

Since I am only willing to support this feature on macOS, I would like to ask what your policy is regarding the addition of features that don't work cross-platform before submitting a pull request.

Best regards, Adrian.