Add support for mission objectives

Objectives can be either primary or secondary objectives. Primary ones
influence the outcome of the game.  If all primary objectives are
completed the game is won, and lost when any of them fails.

Objectives can be added at any stage during the game, allowing to react
dynamically to game events.

The objectives backend only contains the information about the
objectives themselves. It does not check if objectives are completed or
failed.  Instead, the state of objectives must be manually marked. The
backend, however, does check whether the game is won or lost.

OpenRA.Game/World.cs

@@ -42,8 +42,19 @@ namespace OpenRA
 		public void AddPlayer(Player p) { Players.Add(p); }
 		public Player LocalPlayer { get; private set; }
 
-		Player renderPlayer;
+		public event Action GameOver = () => { };
+		bool gameOver;
+		public void EndGame()
+		{
+			if (!gameOver)
+			{
+				gameOver = true;
+				GameOver();
+			}
+		}
+
 		public bool ObserveAfterWinOrLose;
+		Player renderPlayer;
 		public Player RenderPlayer
 		{
 			get { return renderPlayer == null || (ObserveAfterWinOrLose && renderPlayer.WinState != WinState.Undefined) ? null : renderPlayer; }