Slot Viewer changes: (1) Omit Mass Bal Slots, (2) Click on Slot Icon: Show Open Slot dlg
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 7.2

(1) Mass Balance Summary Slots are no longer supported in the Slot Viewer.
    They are shown only in their custom open slot dialog.

(2) In the Slot Viewer, when exactly one slot is shown or selected, clicking 
    on the Slot Icon (a QLabel) shows the Open Slot dialog for that single 
    slot.  In that state (exactly one slot), the Slot Icon shows this tooltip:
    "Click to show in Slot Dialog...".

New utility class:
   class SlotQtDlg::ClickableLabel : public QLabel;

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