Manually set starting airport for CPU players

If you are the first human player, you can edit the CPU opponents.

There was a bit of background work to this, as previously CPU opponents were virtually connected clients. So the server could not tell them apart from real humans. That was neat, but was a bit annoying with things like this, where to the server it would look like a hacker trying to edit another player. Also it would be a problem in the future with adding CPU opponents to multiplayer and mixing virtual and real clients.

So now they are companies created and managed on the server, and flagged as CPU.

Also these CPU opponents would set ready right away, which would then conflict if the player would try to set their starting airport later. So now CPU opponents will wait until all the humans have finished before taking their actions.