Allow atlas and random maps to set the UnitAI stance

[elexis]

We have a couple of maps that spawn with gaia soldiers.

Since they are aggressive by default and will follow anyone they see carelessly into fortifications, maps want to set that to default. This way they attack nearby player units but retreat if these retreat too.

Danubius and Elephantine are two maps that do this using a triggerscript. But it shouldn't be needed to do this.

Atlas users should be able to set it as a property in the UI. Random map scripts should be able to pass it as an argument (maybe a new prototype) when placing the entity.