Allow to choose faction before launching a game + configurable spawn pool per faction

I added a gui to configure seats before launching a game. You can choose faction and set the players on any available seat. Only the server can setup the seats.

In the level, for each seat, you can choose "Choice" to allow the player to choose or you can force one of the available factions (defined in factions.cfg).
For player type, you can choose between "Choice", "Human", "AI" or "Inactive" (pretty explicit)

Note that since the spawn pool is now defined in factions.cfg, I've removed them from all the level files.

For @akien-mga , since I know you like to hunt bugs, I've tried to make the seat configuration as final as I could. That means that during seat configuration, players can be added/can disconnect. In this case they should be added/removed from the combos.
Moreover, I've added network communication so that the clients see in real time how the server is configuring the seats.

Note that there can be more players connected than seats available or used. In any case, connected players that were not chosen for a game will get disconnected and return to main menu.

Note that in the skirmish empty level, you can launch the game with only AI players. In this case, like any not chosen player, you will get disconnected and return to main menu ^^