This allows one to specify which diplomatic entities are allowed to garrison in a garrison holder. This means one could also say that a building may only be garrisoned by the player itself, not its allies.
Some useful behaviour when garrisoning an enemy building will be left to the ones who want to have that.
This kind of depends on D2343 as we want to be able to ungarrison our units from a non-allied building.
- Rework the selection panel to show up. (See the ToDo there.)
- Allow enemies to issue orders to structures (to unload own entities). Or better: unload by tasking the entities to>
- Give CP to owner of the unit (after D4679, I guess).