Removing/Adding strand causes strandSet inconsistency

[leemi]

1. Open Maya
2. Open Script Editor
3. Click CadNano button
4. Add a scaffold strand
5. Delete the strand in 2D view
6. Add the scaffold strand back
7. Resize the strand

# Traceback (most recent call last):
#   File "c:\git\cadnano2\views\pathview\strand\endpointitem.py", line 200, in mousePressEvent
#     getattr(self, toolMethodName)(modifiers, event, self.idx())  # call tool method
#   File "c:\git\cadnano2\views\pathview\strand\endpointitem.py", line 345, in selectToolMousePress
#     selectionGroup.processPendingToAddList()
#   File "c:\git\cadnano2\views\pathview\pathselection.py", line 123, in processPendingToAddList
#     item.modelSelect(doc)
#   File "c:\git\cadnano2\views\pathview\strand\endpointitem.py", line 488, in modelSelect
#     test = document.isModelStrandSelected(strand)
#   File "c:\git\cadnano2\model\document.py", line 147, in isModelStrandSelected
#     sS = strand.strandSet()
# AttributeError: 'NoneType' object has no attribute 'strandSet'
# Traceback (most recent call last):
#   File "c:\git\cadnano2\views\customqgraphicsview.py", line 406, in mouseReleaseEvent
#     self._selectionLock.processPendingToAddList()
#   File "c:\git\cadnano2\views\pathview\pathselection.py", line 123, in processPendingToAddList
#     item.modelSelect(doc)
#   File "c:\git\cadnano2\views\pathview\strand\endpointitem.py", line 488, in modelSelect
#     test = document.isModelStrandSelected(strand)
#   File "c:\git\cadnano2\model\document.py", line 147, in isModelStrandSelected
#     sS = strand.strandSet()
# AttributeError: 'NoneType' object has no attribute 'strandSet'

[grinner]

Fixed with the addition of documentClearSelectionsSignal in document.py. This ensures that hanging callbacks are not called in the views. Perhaps those callbacks shouldn't exist in the first place, but we can resolve that separately and I think this is a good redundancy.