Fix: Slot Viewer was allowing text series slots to be configured [6012]
Bug Number: 6012
Release notes (y/n): No (new functionality)
For Release Nums: 7.2

Gnats 6012: Slot viewer allows text series slots to be configured.

I used this opportunity to prevent a Text slot's unit type from being 
changed, for GUI clients that make use of this utility method:

   bool SlotGUIUtils::isUnitTypeUserConfigurable (
      const Slot* slotPtr, int slotCol=0);

I also hid the Slot Config Dialog's NumDisplayAttributes panel when 
used for Text slots.

The high level fix was preventing the Slot Config Dialog from being shown 
by disabling the Slot Viewer's "View >> Configure" menu item when the 
single selected item is a Text Slot.  See changes in method SlotQtDlg::
sensitizeOperations(), making use of this new method:

   bool SlotQtDlg::firstSelColIsText() const;

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