Disconnection handling if the host leaves

[MrPeteypoo]

Currently when the host disconnects all clients continue playing but can't really do anything, it would be preferable if they were forced to quit the game by the GUI.

[MrPeteypoo]

Commit https://github.com/doctorspoof/SpaceShooter/commit/a40212371359b6d5003daaff4981b9af7049760f implements the feature but can sometimes prematurely detect disconnection upon starting a network game.

[doctorspoof]

Commit https://github.com/doctorspoof/SpaceShooter/commit/b3a81954a37c1bb335cf07422b8f16e2766b264c fixes the premature disconnect detection issue