Provide option to disable AI

[akien-mga]

As suggested on the forums by bark: http://forum.freegamedev.net/viewtopic.php?f=36&t=6791#p67642

Play alone / Sandbox / disabling AI
I'd like to be able to remove all AI's and play the map alone. On any map, without having to open it in the map editor and change it, removing the AI's. I'd like to be able to do this at the game creation dialog box if it isn't too much of a hassle.

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I'd suggest to add an option to make a seat "non playing" in any given map (unless specified otherwise in the level config, as we might want to have some scenarios with mandatory seats). Such a disabled seat would not be created on the server, and its buildings would be attributed to seat 0. So its portal could still be claimed by players for example.

[hwoarangmy]

Note that in theory, rogue portals can spawn creatures. They usually don't because rogue seat will almost always have more creatures than the minimum it can own (5 x number portals). I think it would be better (and simple) to not allow rogue portals to spawn creatures. WDYT ?