#4981 Fix well target manipulators not responding

Moved z scale change notification to method setting zscale for label
Was always done, and recreated target manipulators at first mousemove
Avoid updating cached values in scene on repeated calls to show gridbox

ApplicationCode/UserInterface/RiuViewer.h

@@ -175,6 +175,7 @@ private:
     QLabel* m_zScaleLabel;
     bool    m_showZScaleLabel;
     bool    m_hideZScaleCheckbox;
+    double  m_zScale;
 
     caf::QStyledProgressBar*  m_animationProgress;
     bool                      m_showAnimProgress;