+ Refactor item-based attribute classes.
  Move most of the code in ComponentItem, ResourceItem,
  ComponentItemDefinition, and ResourceItemDefinition into
  two new base classes LinkItem and LinkItemDefinition.
  These classes handle links between an attribute and
  any resource or component of a resource.
  Note that smtk::attribute::RefItem should disappear
  in favor of a ComponentItem whose `acceptableEntries()`
  filter forces an attribute of the proper type.
+ Fix build when `SMTK_ENABLE_EXAMPLES` is on.
+ Prototype for a qtComponentItem.