Introduce map Visibility field.

This replaces the Selectable, UseAsShellmap, and special-cased Type = "Mission" fields.

OpenRA.Game/Game.cs

@@ -383,7 +383,7 @@ namespace OpenRA
 		static string ChooseShellmap()
 		{
 			var shellmaps = modData.MapCache
-				.Where(m => m.Status == MapStatus.Available && m.Map.UseAsShellmap)
+				.Where(m => m.Status == MapStatus.Available && m.Map.Visibility.HasFlag(MapVisibility.Shellmap))
 				.Select(m => m.Uid);
 
 			if (!shellmaps.Any())