Slot Viewer: Tear Off Slot Column into Open Slot Dialog
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 7.2

This commit adds the ability to DRAG ("Tear Off") a Slot Column from the 
Slot Viewer.  While dragging, once at a sufficient minimal distance from 
the column headers, the cursor turns into the Slot icon.  In this state, 
when the mouse button is released, the slot is removed from the Slot Viewer, 
and shown in its own Open Slot dialog.

(For the purpose of "redocking" an Open Slot dialog into the Slot Viewer, 
there is some provisional work for dragging the Slot Column from an Open 
Slot dialog.  But I'm thinking that this should instead by performed by 
dragging the Open Slot Dialog's SLOT ICON.  That would be consistent with 
what we did with the new Object Viewer and Open Object dialogs).

New SlotQtDlg method:

   void slotViewer_detachSlotCol (SlotColRef, QPoint globalPos);

Most important new SlotDataTableView methods:

   virtual void mousePressEvent (QMouseEvent*) override;
   virtual void mouseMoveEvent (QMouseEvent*) override;
   virtual void mouseReleaseEvent (QMouseEvent*) override;

   bool tearArmedAtPos (const QPoint& pos);
   void updateOverrideCursor();

New SlotDataTableView fields involved in this mechanism:

   bool _dragSlotHeaderActive;
   bool _dragSlotHeaderArmed;
   SlotColRef _dragSlotHeaderSlotColRef;
   QPoint _dragSlotHeaderStartPos;
   QCursor _dragSlotHeaderCursor;
   bool _dragSlotHeaderOverrideCursorSet;

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