Extensions should support Fragment in-app predictive back animations

[Zhuinden]

https://github.com/Zhuinden/simple-stack-extensions
With Fragment 2.7.0 out, that only supports predictive back when using fragmentManager.addToBackStack() requires creating a new version of the DefaultFragmentStateChanger, that would hopefully seamlessly micromanage the transactions that get the UI to be in the expected state.

It might seem a bit scary at first, but it'll have to be done sooner or later.

See https://developer.android.com/jetpack/androidx/releases/fragment#1.7.0