Gnats 4647: Clearing workspace with slot with expression dialog open crashes
This crash occurred on Windows, both 5.1 and 5.0.4. Not on Solaris.
Fixed 11-17-2008, in both builds and prerel.

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Q3GUI/RplExprSlotDlg.cpp
Q3GUI/RplExprSlotDlg.hpp
Q3GUI/Workspace.cpp
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Three changes:

(1) Workspace::closeWorkspaceObjDialogs(), use delete dlg; instead of
  close(true); On Windows, the effect of deleting Open Object dialogs
  was being decoupled, which broke our 'clearing' mechanism.

(2) Needed QObject 'destroyed()' notification from a RplExprSlotDlg's
  RplQtFrame to the dialog.  The RplDlgMgr was just clearing its RplFrameList
  without notifications to the owners of a RplFrame (and then it asserts
  when a client tries to remove its RplFrame!).

(3) Safer handling of Slot and SimObj deletion callbacks within
  RplExprSlotDlg.

Change processing in:
 (a) RplExprSlotDlg::simObjDeletedHandler (SimWorkspace*, SimObj*)
 (b) RplExprSlotDlg::slotDeletedHandler (SimObj*, Slot*)

OLD (first example):

   if (simObj && (_simObj == simObj))
   {
      deleteCallbacks();
      _simObj = NULL;
      close (true);
   }

NEW (first example):

   if (simObj && (_simObj == simObj))
   {
      hide();
      deleteCallbacks();

      if (_rplFrame)
      {
         delete _rplFrame;
         _rplFrame = NULL;
      }

      _simObj = NULL;
      _slot = NULL;
      close (true);
   }

Also, a few _rplFrame NULL pointer checks before dereferencing.

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