This modifies qtReferenceItem to implement link hover highlighting.

Additionally, there are a number of supporting changes:
+ We add a new mode for `smtk::view::Selection::modifySelection()`
  that will apply modifications bitwise. It is not the default, though
  it should become the default
+ Prevent selection updates during selection updates in the
  pqSMTKResourcePanel.
+ In the qtModelEntityAttributeView, do not display a row's attribute
  if it is already being displayed. Doing so causes problems when a
  highlight is added to the selection but the primary selection is
  unchanged, resulting in flickering during mouse hovering over
  qtReferenceItem children of the attribute attached to a model item.
+ Be more careful about selection bit values in several places where
  previously selection-map membership alone was being tested or where
  0 values were written to a selection-map (when in fact that should
  result in the map entry being erased).