Settings persistence and allow changing language

Also partly covers #196 (closed) by introducing a Settings interface where all settings are to be managed, but I didn't integrate those yet with the FeatureFlags, nor did I provide them via LocalCompositionProvider since it is not needed for the two settings we are currently supporting. If you think it's a good idea, I could certainly add another commit where I fully address #196 (closed) as well.

Closes #212 (closed)