Scenario Playbox: "Disable for Scenario" completion; Also Scenario Slot "Diff" Fixes.
Bug Number: n/a
Release notes (y/n): No
For Release Nums: 6.3

(1) Serialization of the new Scenario '_playboxEnabled' flag, including 
initialization logic to correctly accommodate the loading of old Scenario 
files.

(2) Fixes to the Scenario Manager's indication of Scenario Slot change status.
It turned out that loaded Scenarios incorrectly had "changes" indicated for ALL 
Scenario Input Slots (if they had any data).  The "conservative" implementation
indicated changes whenever any value was assigned to these slots, which sometimes
indicates a 'false' change state -- generally OK.  However, this is undesirable 
when values on these slots are set simply because they are being loaded from a 
Scenario file.  The fix makes use of these new utilities:

(3) New Slot utility functions in SlotGUIUtils to compare the values of slots
which are intended to be potentially-modified copies of each other. This is
used in the Scenario mechanism to determine whether a Scenario Slot has any 
changes relative to it's source Baseline Slot.

  bool slotsHaveEqualValues (const Slot*, const Slot*, Date_Time* firstDiffRet=NULL);
  bool slotsAreCongruent (const Slot*, const Slot*);
  bool seriesSlotsAreCongruent (const SeriesSlot* ss1, const SeriesSlot* ss2);
  
... see first use in Scenario::recomputeInputChangeStatus().

(4) The conditionally shown "Enabling Scenario Playbox" WARNING popup is positioned  
BELOW the Scenario Manager's (enable-) Playbox checkbox.  Not obscuring that 
checkbox makes the condition explanation easier to understand.

(5) Scenario Playbox Output Slot List context menu operations: 

   (a) Show in New SCT ...
   (b) Add to Open SCT ...

... this adds the version of the selected Output Slot from EACH of the selected 
Scenarios (Snapshot Objects) to an SCT.  The "Add to Open SCT..." operation is 
enabled only when exactly one SCT is shown (not minimized).

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