Disconnect inactive players

[filipbartek]

Inactive player should automatically leave the game. After a period of inactivity (e.g. 5 seconds), the player simply disappears and the controller is available for joining the game again.

Possible to combine with smoking characters: see issue #18

[filipbartek]

Watch out for unfairness: a user who's trying to disconnect from the game by staying idle may give unfair advantage to some of the other players.