Slot Viewer: Column width stability in operations which change the set of slots
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 7.2

When deleting or adding slots to the Slot Viewer, the default sizes were 
assigned to all of the data columns.  We now retain that column width 
information, and reapply that when the QTableView header is reconstructed 
with the new slot list.

In Q3GUI/SlotDataTableModel, sww the use of these provisions:

   QMap<SlotColRef, int> _slotViewer_colWidthMap;
   void recordSlotCol_colWidthMap (QMap<SlotColRef,int>&) const;
   void applySlotCol_colWidthMap (const QMap<SlotColRef,int>&);

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