Add bkNotebookCellModelManager to bkDebug
Fix #235, make delete cell undoable

After a cell is deleted, use Ctrl + z to undo. The history buffer size
is 1. Also any other cell operation that involves recreating cell map,
e.g. move up/down will clear the history buffer and void undo.
Merge pull request #606 from twosigma/alee/fix-235

Make delete cell undoable