Set up Jetpack Compose

[westnordost]

Other than adding the dependency, the (dark+light) theme, style, colors, fonts are defined in code in Jetpack Compose rather than in res/values/*.xml files.

So, as a start, the current theme etc should be duplicated there. (See any sample Compose app for an example).

The app uses a theme based on Material 2. If a migration to Material You (=3) should happen, the best time would be to do this before this ticket, as the theme definition also for compose looks a bit different between Material 2 and 3.

I've tried it before, but ended up not doing it, see #3650. (I have no really strong opinion on not using Material You - if someone else did the migration, I'd be okay with it)

[westnordost]

#5607