Move Game.Timestep to Widget

Game.Timestep wasn't used for anything other than
UI anymore anyway, moving it makes this more clear.

OpenRA.Game/Widgets/Widget.cs

@@ -20,6 +20,8 @@ namespace OpenRA.Widgets
 {
 	public static class Ui
 	{
+		public const int Timestep = 40;
+
 		public static Widget Root = new ContainerWidget();
 
 		public static long LastTickTime = Game.RunTime;