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/Traits/World/EditorActorLayer.cs

@@ -140,6 +140,8 @@ namespace OpenRA.Mods.Common.Traits
 			foreach (var cell in footprint)
 				AddPreviewLocation(preview, cell);
 
+			preview.AddedToEditor();
+
 			if (!initialSetup)
 			{
 				UpdateNeighbours(preview.Footprint);
@@ -171,6 +173,7 @@ namespace OpenRA.Mods.Common.Traits
 					cellMap.Remove(cell);
 			}
 
+			preview.RemovedFromEditor();
 			UpdateNeighbours(preview.Footprint);
 
 			if (preview.Info.Name == "mpspawn")