Allow to set rally points for allied buildings

[Kalle Richter]

This would allow to determine the place where garrisoned units would go after being ungarrisoned. Of course, this has nothing to do with the rally point the player who owns the building, i.e. a rally point for each allied player would have to be provided. An example use case would be a row of allied defence towers where the first one has some of your ranged infantry garrisoned. You'd want to set the rally point of the first tower on the second (so that unit would be garrisoned there), the one of the second on the third, etc.