Slot Viewer: Redocking of Open Slot Dialogs in Slot Viewer (drag slot icon)
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 7.2

This commit provides redocking of an Open Slot Dialog into the Slot Viewer 
-- by dragging the Open Slot Dialog's slot icon.  The drag icon turns into 
the slot's icon when it enters the Slot Viewer.  If dropping (releasing the 
mouse button) in the Slot Viewer, a display column is created for that slot 
AT the horizontal position of the drop point and the original Open Slot 
Dialog is closed.

See new class coded in Q3GUI/SlotQtDlg:

class SlotIconButton : public QPushButton
... this emits this Qt signal:
   void dropped (const QString& fullSlotName, QPoint globalPos);

See also SlotQtDlg methods:

bool slotViewer_slotDragDropped (const QString& slotName, QPoint globalPos);
 ... used in Slot Viewer

void slotIconButton_dropped (const QString& slotName, QPoint globalPos);
 ... used in Open Slot Dialog

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