allow gradual transition form `MaterialApp` to `MacosApp`

[stephane-archer]

Having to place MacosApp instead of MaterialApp doesn't allow you to gradually transition from a Material app to a MacosApp, making complex apps impossible to transition.

While you could certainly use macos_ui widgets and themes without using a MacosApp, it would be quite messy to do so

while the document states it is possible, I have no idea how to do this and it clearly states that it is a bad idea.

[Adrian-Samoticha]

It might be possible to wrap your app in both a MaterialApp and a MacosApp, though I haven’t checked on that. This way you’d be able to gradually transition and remove the MaterialApp once you’re done.

[stephane-archer]

@Adrian-Samoticha I tried but it didn't seem to work (but I didn't insist much)