1.5.7

1. April 2022 exoplanet update
2. Support split screen
3. Use web browser for add-on list and news archive
4. Rearrange some setting options
5. Fix brightness issue on rendering nebulae and other objects

celestiamobile/Celestia@5a12452
CelestiaProject/CelestiaContent@7f816cc
celestiamobile/CelestiaLocalization@d569a80