Allow map designers to "hide" player slots.

Oftentimes when hosting a custom map, we need certain players to be CPU. Usually it would be something like "last player MUST be CPU!" or "last 2 players MUST be CPU!"

We would like a feature that allows us to add hidden players to our custom maps that do not show up as available slots in the pre-game lobby or are already filled with computers.

Alternatively, add a function that allows us to add a computer player on game startup, which can achieve the same result.