Add a button to undo the saving of cleaving

[hubbardp]

When the user checks "completed" in the Neu3 cleave tool, the DVID cleave API will be called, which will apply the cleaving to the segmentation. If the user wants to change the cleaving of that body, the user must first undo the cleaving by calling the DVID merge API. The cleave tool should have a button to make this call.

This work will use the updated information in the "_cleaved" key/value instance described in this task:
#97