Allow to set collection for allied buildings

[Kalle Richter]

This would allow to determine the place where quartered units would go after being unquartered. Of course, this has nothing to do with the collection point the player who owns the building sets, i.e. a collection 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 quarters some of your distance infantry. You'd want to set the collection point of the first tower on the second (so that unit would be quartered there), the one of the second on the third, etc.