Slot Viewer: Some "Slot Description" support for Slot Dialog Display Prefs.
Bug Number: n/a
Release notes (y/n): no (tweak to new functionality).
For Release Nums: 7.2

Fix to dysfunction Mitch found with the Slot Viewer's support for the 
"Show Slot Description" setting in Slot Dialog Display Preferences. 
See this preferences dialog:

http://cadswes2.colorado.edu/~philw/2017/ObjSlotViewerRev/Ref/SlotDialogDisplayPrefsDlg.png

When starting off the Slot Viewer with a single slot -- and the initial Slot 
Description visibility setting is "Show if slot has description" -- we would 
expect the Slot Description panel to be shown if that slot has a description.
That was not functional, for two reasons:

  (1) The implementation of SlotQtDlg::applySlotDlgPrefs() was not complete 
      with respect to Slot Viewer mode.

  (2) That method was being called too early, before slots were inserted into 
      the Slot Viewer.

When in Slot Viewer mode, the applySlotDlgPrefs() method now correctly refers 
to either the Slot Viewer's single selected slot OR the first slot -- that is,
for a determination of whether a non-empty slot description is present.  Also,
that method is now also called from SlotQtDlg::slotViewer_addSlots() if the 
Slot Viewer had been empty, but has become non-empty.

Note that the Slot Viewer (in RiverWare 7.2) does not currently support the 
showing of the Slot Notes column, so the corresponding preferences setting 
is not (yet) relevant.

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