Assign multiple airlines to a stand

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).

I agree on this.

This is one of none-essential things that I would really like to see as well.

In a lot of medium airports members of same airline alliance use same set of gates, while all the low cost carriers also use each other gates. Would be nice to have this implemented.