Assign multiple airlines per gate

Feature request title:

Assign multiple Airlines per gate

Description:

At the moment you can either allow all airlines at a gate or assign a single airline for this specific gate. It should be possible to allow multiple airlines for a gate (via checkbox).

Why it should be implemented:

For realism. At real airports you have often some Terminals reserved for a special airline alliance, like Skyteam, Oneworld or Star Alliance.
In this Terminal you then have many airlines from that alliance at the gates.
You can not play with such alliance terminals with automatic flight planning at Airport CEO at the moment. You have to manually assign the planes to the gates for this. Otherwise you can only have one gate assigned to a single airline. So when I have 6 gates and 6 different airlines for One world terminal, with automatic flight planning and assigned gates each airline can only use one dedicated gate at the moment. So an airline can not land 2 planes at the same time, even if 5/6 gates would be free at that time.

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