Add "Quit campaign" / "Quit current game" button to Singleplayer and Multiplayer...

Add "Quit campaign" / "Quit current game" button to Singleplayer and Multiplayer / Create windows to properly quit campaign, and also disable the "Start Multiplayer!" button while playing the campaign.

This way it's no longer possible to start a multiplayer game while the campaign settings are still visible in Multiplayer / Create window (these settings are temporary and won't be applied anyway)