Slot Viewer: Close Column Red-X button: Make Grey EXCEPT when Hovering over button
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 7.2

We are now using a QPushButton SUBCLASS so that we can change its icon 
(color) when the mouse Enters or Leaves the button area.  What had been 
"Red-X" icon buttons are now Grey, except when the mouse is hovering over 
the button, in which case the "X" button will be Red.

New class:
  class SlotDataTableView::CloseColBut : QPushButton

Overridden virtual methods (from QWidget):
   virtual void enterEvent (QEvent*) override;
   virtual void leaveEvent (QEvent*) override;

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