+ Add the qtReferenceItem subclass of qtItem. Then add
  qtComponentItem which inherits qtReferenceItem. Eventually,
  qtResourceItem will also exist (or qtReference item will
  encompass functionality for both resources and their components).
    + This item now properly presents and allows editing
      of entries related to a ComponentItem. However, it does
      not update the entries of the ComponentItem upon editing yet.
    + The Qt item displays a synopsis of the selected components
      and indicates whether the selection meets the requirements
      called out by the attribute (specifically, the number of
      entries must be allowed and the selection must come from a
      list of acceptable components).
+ Add callbacks to SMTK resource panel to signal when a row is hovered
  and change the "hovered" selection value.
+ Add support to the model representation to illustrate the "hover"
  selection value in preference to the "selected" value.
+ Fix a memory leak in the model representation.
+ Fix the SMTK attribute panel exposed as a PV plugin:
    + Always clear the widget out when switching between
      attribute resources.
    + Note that this has not been tested with switching between
      views within the same attribute resource, and will probably
      need changes to handle that case.
    + Also note that you must click on a _resource_ entry in the
      resource tree-view panel to switch what is displayed in the
      attribute panel; clicking on an attribute is not enough.