Undo/redo should not re-render unchanged objects

[bjnortier]

In the graph replacement algorithm this used to be the code:

  var fromHash = fromGraph.hashForId(id);
  var toHash = toGraph.hashForId(id);
    if (fromHash !== toHash) {
      listener({vertexReplaced: {from: fromGraph.get(id), to: toGraph.get(id)}})
    }
  }

but when the hashes are the same, the vertices in the view models need to be replaced so the views will react properly to 'change' events, especially when an implicit
point has been replaced during undo/redo (see bug #12 )

So now all vertices are replaced on undo/redo which leads to flashing, which is not ideal.

So either the vertices must be replaced in the display models or another mechnism must be found to remove the flashing