Reorganize actor and smudge loading.

The actor and smudge definitions are now stored
as raw MiniYamlNodes in the map.  It is now the
responsibility of the consumers to parse these
into real objects.

OpenRA.Game/Map/MapPreview.cs

@@ -119,7 +119,7 @@ namespace OpenRA
 			Type = m.Type;
 			Author = m.Author;
 			Bounds = m.Bounds;
-			SpawnPoints = m.GetSpawnPoints();
+			SpawnPoints = m.SpawnPoints.Value;
 			CustomPreview = m.CustomPreview;
 			Status = MapStatus.Available;
 			Class = classification;