Add Selectable field to Selectable.

There are a bunch of bogus assumptions about
targetable actors being selectable. These aren't
easily fixed, so this add a Selectable field that
can be diabled for things we want to target, but
not select.

OpenRA.Game/Widgets/WorldInteractionControllerWidget.cs

@@ -184,7 +184,7 @@ namespace OpenRA.Widgets
 		IEnumerable<Actor> SelectActorsInBox(World world, PPos a, PPos b, Func<Actor, bool> cond)
 		{
 			return world.FindUnits(a, b)
-				.Where(x => x.HasTrait<Selectable>() && !world.FogObscures(x) && cond(x))
+				.Where(x => x.HasTrait<Selectable>() && x.Trait<Selectable>().Info.Selectable && !world.FogObscures(x) && cond(x))
 				.GroupBy(x => x.GetSelectionPriority())
 				.OrderByDescending(g => g.Key)
 				.Select(g => g.AsEnumerable())