Workspace View Canvas Rectangle [2]
Bug Number: n/a
Release notes (y/n): no
For Release Nums: 7.3

Development for a visible rectangle showing the extent of the view canvas size.
This is being committed to a new temporary branch: ViewRect2.

This is the second provisional implementation of this feature.  We originally 
tried deploying a QGraphicsRectItem of exactly the QGraphicsScene's "scene 
rectangle".  That's what we do in the Output Canvas (to show the configured 
bounds).

It turns out though that, in the RiverWare workspace, because of the 
implementation of background images, there is no way of showing a 
QGraphicsItem underneath the background image.  SO instead, we are drawing 
the canvas rectangle in this method: RwGraphicsScene:: drawBackground(), 
a virtual method from QGraphicsScene.  This is actually a lower-complexity 
solution, as there is no way that mouse events will be interacting with the 
drawn canvas rectangle.

The ability to hide the canvas rectangle was implemented, but there is 
currently no way (GUI control, or anything) to turn it off.  This is 
implemented with these new RwGraphicsScene members:

  bool _showSceneRect;
  bool showSceneRect() const { return _showSceneRect; }
  void setShowSceneRect (bool doShow);

When the scene rectangle is SHOWN, it is drawn with the desired (configurable) 
background color, and the QGraphicsScene's actual background color is set to 
be 40% darker.  When the scene rectangle is HIDDEN, the desired background 
color is assigned directly to the QGraphicsScene's actual background color 
(as had been done before this feature was implemented).  This makes use of 
these new RwGraphicsScene members:

  QColor _sceneBgColor;
  QColor sceneBgColor() const { return _sceneBgColor; }
  void updateSceneBgColor();  // private

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