Restore Alt-f4 quit shortcut for Windows users.

OpenRA.Renderer.SdlCommon/SdlInput.cs

@@ -115,6 +115,13 @@ namespace OpenRA.Renderer.SdlCommon
 							MultiTapCount = MultiTapDetection.DetectFromKeyboard(keyName)
 						};
 
+						// Special case workaround for windows users
+						if (e.key.keysym.sym == Sdl.SDLK_F4 && mods.HasModifier(Modifiers.Alt) &&
+						    Platform.CurrentPlatform == PlatformType.Windows)
+						{
+							OpenRA.Game.Exit();
+						}
+						else
 							inputHandler.OnKeyInput(keyEvent);
 
 						break;