Feature request title:
Assign multiple airlines to a stand
A while ago assignment of an airline to a stand was introduced, however it is only possible to assign a single airline to a stand and not several. This should be changed to allow multiple assignment.
Why it should be implemented:
With multiple assignments players can simulate airline alliances, for example allowing a group of 10 airlines to use the stands of one terminal area and a group of 5 other airlines to use the stands of another terminal area.
Images, references or additional content:
A similar possibility already exists for staff rooms where you have multiple choice for the staff types:
Airline-to-stand assignment should look the same (just with scrollbar in case of a too long list of airlines; the list should be ordered alphabetical).