Slot Viewer: Open slots in Viewer; Description panel; Various other features, fixes.
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 7.2

Functionality completed in this commit:

 (1) Open series slots in viewer (instead of, initially, in their 
     Open Slot dialogs).

 (2) Slot description panel content support -- needs to be repopulated each 
     time the single selected slot changes -- and erased when more than one 
     slot is selected.

 (3) Better formulation of column header text (for slots).
 (4) Slot header context menu, with "Show in slot dialog".
 (5) Fix: "Show in SCT" needs to apply visible (user reordered) order.
 (6) Various technical fixes.

IN PROGRESS:

 (7) Column width stability in operations which change the set of Slots 
     in the viewer.

STILL TO DO:

 (8) Fix timestep synchronization bug involving AggSeriesSlots having 
     columns starting at different times, of high level value editing 
     operations, e.g. "Fill Below".

 (9) Slot callback registrations for all columns (slots).  In particular, 
     the lack of servicing of the SLOT DELETION callback is currently 
     crashing.

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