Short Description: Implemented Import/Export in Scalar Slot dialog.
Bug Number: 4133
Release notes (y/n): Yes
For Release Nums: 5.3

Gnats 4133: Scalar slot dialog should have import export functionality
SEE SCREENSHOTS: https://cadswes2.colorado.edu/~philw/2010/bugs/4133/ 

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Q3GUI/ScalarSlotDlg.hpp
Q3GUI/ScalarSlotDlg.cpp
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Added File >> Import and Export operations, for the dialog's Scalar Slot
value.  Only "model precision" (and not "display precision") is supported 
(implicitly) for Scalar Slots.

New data members and methods:
  QAction* _importValueAction;  // _fileMenu
  QAction* _exportValueAction;  // _fileMenu
  void importValue_activated();
  void exportValue_activated();

Import errors detected and reported:
 (1) File is compressed (see below).
 (2) Unable to open file for reading.
 (3) Low-level import and numeric conversion errors.

Export errors detected and reported:
 (1) Query user for permission to overwrite existing file.
 (2) Unable to open file for writing, e.g. write-protection.

Note: We were CRASHING when trying to import from a gzipped (compressed)
file. (This is true also for the existing functionality of importing data 
into Series Slots -- I'll file a bug for that).  But for Scalar Slots, 
we'll just not support importing from a compressed file.  

-------------------
Q3GUI/SlotQtDlg.hpp
Q3GUI/SlotQtDlg.cpp
-------------------

New public static method:
  static QString chooserPath();

.. provides access to the "cached" recently-accessed file path value, 
for slot import/export operations.

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