Slot Viewer: Fix to preservation of reordered column order
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 7.2

Certain operations (e.g. providing a value for a slot/timestep cell which 
results in extending the slot's series time range) were causing the 
user-adjusted column order to be lost.  This was because such operations 
were causing a recomputation of the QTableView's row/column layout -- 
with a call to SlotDataTableModel::recomputeCellLayout().  When that happens 
we need to reorder the "logical" slot order using the current "visual" 
column order.  This is now done right in that method.

See also revision to the following method, which performs that reordering.
It is now "non-destructive" of the ordering of the original list's slots 
which are not yet present in the QTableView.

void SlotDataTableView::sortSlotListByVisualIndex
   (QList<SlotColRef>& slotList) const

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