SCT: Support for editing Text Series Slots
Bug Number: n/a
Release notes (y/n): Yes
For Release Nums: 7.2

Text Series Slots were introduced for RiverWare 7.1.  But they were editable 
only in the Open Slot dialog -- not in the SCT.  This commit adds SCT support 
for editing Text Series Slots, including the setting of a multiple cell 
selection either by just starting to type, or entering text in the main 
QLineEdit at the top of the SCT.

It's important that SCT operations which set numeric values be disabled when 
Text Series Slots are part of the selection.  The standard series slot values 
setting mechanisms DO transparently function when operating on Text Series 
Slots -- however, in general, create an INVALID VALUE STATE which, generally 
fails assertions.  So we need to be careful to disable all such operations 
when Text Series Slot timesteps (cells) are selected.

New methods:
   bool SctTableModel::isTextSlot (int slotIndex) const;
   bool SctModelData::isTextSlot (int slotIndex) const;

   bool SctSlotTstepSet::hasOnlyTextSlots (
      int* textSlotCntRet=NULL,
      int* numSlotCntRet=NULL) const;

   okstat SctModelData::setSingleTstepTextValue (
      int slotInx, int tinx, const QString& str);

   void SctDialog::setTextValueOnSlotTstepSet (
      SctSlotTstepSet*, const QString&);

Visitor Class Changes and Additions:
(these are implementations of SctSlotTstepSet::ForEachFunction).
   (1) SctDialog::ValueSetter renamed to SctDialog::NumValueSetter.
   (2) Added: SctDialog::TextValueSetter.

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