Allow vehicles as leaders of a simulated region

[lukasrad02]

Some kinds of simulated regions, for example staging areas, are not commanded by a single person but a vehicle (e.g. a KdoW/command vehicle). This should be supported by the simulation, either by a configuration option on the assign leader behavior or by an additional assign command vehicle behavior.

For realistic scenarios, command vehicle templates (KdoW, ELW) should be available. The behavior should prefer command vehicles over other vehicles and exchange non-command-vehicles if a command vehicle becomes available.