simutrans: update, refactor, improve, add paksets

Close #7019.

- update all, and add more paksets
- add config.simutrans.paksets, multiple are possible now
- fix #6719: missing sounds
- move user settings from ~/simutrans to ~/.simutrans
- darwin support is untested (but claimed upstream)

Tested-by: Matthias Beyer <mail@beyermatthias.de>
(and by the author vcunat)