Don't minimize the game when tabbing out

[Skamandros]

With the new full screen mode, the game is minimized when the user uses Alt+Tab to change to another window. This would prevent opening the game on a second screen and doing other stuff while waiting for a game (which would also require out-of-focus GUI refresh).

[Matthias Kricke]

fixed

[Skamandros]

Removing milestone: Lange Nacht (automated comment)