[12677] Make use of GROUP_UPDATE_FLAG_VEHICLE_SEAT

Original patch by thenecromancer (TC)
[12678] Fix possible client crash when passenger despawns while boarded
[12679] Revert [12646]. This needs to be fixed in another way!

Close scriptdev2/scriptdev2#113
[12680] Improve [12678] with additional Vehicle checkes

This is required because transports are not always vehicles