Add "idle" command to entity commands

[Versepelles]

I'd like to request that defines.command.idle be added to defines.command. The idle command makes the entity stay in place, rather than wander (defines.command.wander). This would be very useful for mods trying to control AI behavior (see Alien Farm, Robot Army). I have a Pikmin-style mod in mind, but it would be difficult without this feature.

[Bilka]

This function is fulfilled by defines.command.stop