Undo delete a 'box' should update graph animators

[rodlie]

Issue found when fixing removal of graph animators in #178

When you undo a deleted box the graph animators are not updated, this is because the canvas does not add the box back to selected items. Thereby the graph will not show the animators for the undo'ed (it that even a real word) box until you click it.

So, we should make sure that the canvas adds the box back to selected items when undo.

This issue will only manifest itself in #178, as this was never a problem earlier because we never removed any graph animators in the first place.