Make Conquest- and StrategicVictoryConditions use objectives backend

Note: 3rd party KotH maps will need to be modified to work correctly
with this change.  Previously, StrategicVictoryConditions was a
supplement to ConquestVictoryConditions.  After this change, each works
by itself.  So KotH maps will need to remove the ConquestVictoryConditions
trait from the player definitions, or both victory conditions will have
to be satisfied to win the game.

OpenRA.Mods.RA/Player/Extensions.cs

@@ -0,0 +1,23 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2014 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation. For more information,
+ * see COPYING.
+ */
+#endregion
+
+using System.Linq;
+using OpenRA.Mods.RA;
+
+namespace OpenRA
+{
+	public static class Extensions
+	{
+		public static bool HasNoRequiredUnits(this Player player)
+		{
+			return player.World.ActorsWithTrait<MustBeDestroyed>().All(p => p.Actor.Owner != player);
+		}
+	}
+}