State restoration

[etiennelenhart]

While Eiffel's approach to state handling already helps with restoring state on orientation changes and UI resuming thanks to Architecture Components' ViewModel, another common problem on Android are process kills.
Since ViewModel can't help with this a solution should be investigated to store selected properties of a State using onSaveInstanceState. Especially the new androidx.savedstate library might come in handy. Considering its alpha status, possible other solutions should still be evaluated.

Solutions using reflection are strongly discouraged because of the significant performance overhead and other approaches like compile-time annotation processing should be considered.

[etiennelenhart]

Postponed until androidx.savedstate is stable and its usage clearer.

[etiennelenhart]

Now that androidx.savedstate is in beta and rudimentary documentation is available, this can be implemented.