Added Buildable->BuildLimit option to limit the number of actors a player can build of the type

OpenRA.Mods.RA/Buildable.cs

@@ -22,6 +22,7 @@ namespace OpenRA.Mods.RA
 
 		public readonly string Queue;
 		public readonly bool Hidden = false;
+		public readonly int BuildLimit = 0;
 
 		// todo: UI fluff; doesn't belong here
 		public readonly int BuildPaletteOrder = 9999;