Cut, copy, paste, delete, split and merge

[colinjbrown]

Please see:

dataflownb/dfkernel#20

As things are moved to the notebook extension we should make sure that all of these are handled properly

    Cut+Paste: retain original ids
    Copy+Paste: assign new ids and remap cell references for cells that were copied
    Delete: works as before, might attempt to flag cells the depended on the deleted cell
    Split: bottom cell retains the original id, issue will be that chaining the cells will not work so best practice is to avoid this
    Merge: retain the cell id of the last cell; since order doesn't matter in dataflows, this operation could be problematic

Note that we could try to be more proactive with split and merge. For split, we could add an extra line at the beginning of the bottom cell that references the top cell, and for merge, we could remap cell references in merged cells to local variable references. However, this could introduce issues with what users expect.```