Require undo of cleaving before any changes to a completed task

[hubbardp]

When a user returns to a task with the "completed" box checked, all controls that change the cleaving for that task should be disabled. The spacebar to set (toggle) a seed probably is the only such control.

When the user triggers such.a control, then a dialog will appear, telling the user that the cleaving must be undone before a change can be made. Thus this work depends on the undo button described in this task: https://github.com/janelia-flyem/NeuTu/issues/98