Add INotifyEditorPlacementInfo interface.

This allows TraitInfos to act when the actor preview is placed
in the editor, returning arbitrary data which the editor stores
and gives back if the preview is removed.

OpenRA.Mods.Common/TraitsInterfaces.cs

@@ -590,6 +590,13 @@ namespace OpenRA.Mods.Common.Traits
 		}
 	}
 
+	[RequireExplicitImplementation]
+	public interface INotifyEditorPlacementInfo : ITraitInfoInterface
+	{
+		object AddedToEditor(EditorActorPreview preview, World editorWorld);
+		void RemovedFromEditor(EditorActorPreview preview, World editorWorld, object data);
+	}
+
 	[RequireExplicitImplementation]
 	public interface IPreventMapSpawn
 	{