CoeSwd9: (1) Low-level life-cycle safety, (2) corrected order, (3) renamed Item to PlotItem
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 7.1

(1) When SlotPlots, PlotMarkers and AxisItems get deleted (from the Plot 
Dialog) and related classes, deletion notifications are routed through the 
SlotPlot (a QObject) to the PlotItemTreeView::PlotItem to drop its 
corresponding pointers.  Corresponding items in the PlotItemTreeView 
subsequently show blank cells where the text of those cells had depended 
on those pointers.  This is a low-level safety.  Higher level life-cycle 
mechanisms will subsequently be developed.

(2) The order of PlotItemTreeView::PlotItems in the displayed list was 
backwards.  The internal ordinal fields on which it had been based were 
not valid.  This is fixed.

(3) Inner class PlotItemTreeView::Item was renamed to PlotItemTreeView::
PlotItem so that "greps" for that class would not also find 
"PlotItemTreeView::ItemModel".

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