This is a large, breaking change to SMTK that moves the attribute
system away from using ModelEntityItem. Specifically, it

+ switches `Attribute::associations()` methods to return a
  ReferenceItem instead of a ModelEntityItem.
    + removes m_associations from Attribute.
+ switches `Definition::associationRule()` and related methods
  to return a ReferenceItemDefinition instead of a
  ModelEntityItemDefinition.
    + fix many methods in Definition that referred to m_associationRule
      to use m_acceptsRules instead.
+ updates the XML and JSON readers and writers to
  deserialize ModelEntity items/definitions in old files into their
  equivalent Reference items/definitions in the new system.
+ refactors ReferenceItem to no longer be a template
  since Resource and Component now share a common ancestor.
+ simplifies the qtReferenceItem hierarchy since now both
  Resource and Component inherit PersistentObject.
+ renames `Attribute::isComponentAssociated()` to `isObjectAssociated()`.
+ consolidates a lot of code that was shared between ReferenceItem,
  ResourceItem, and ComponentItem into ReferenceItem.
+ changes the default attribute reader version to 3 (was 2).
+ removes an unneeded argument from `from_json` methods.
  By the time these methods are called, `itemPtr` should already
  own a reference to the collection via its parent attribute, so
  don't pass the collection needlessly.

  This also simplifies the json format for items by eliminating
  the `ItemValue` dictionary; instead, items are responsible for
  adding the `Type` entry to their own dictionaries.
  It may even be possible to remove the `Type` entry since the
  items are required to have their type specified by a definition.
+ makes OperationEnvironment depend on AttributeEnvironment.
  Operations depend on attributes for their parameters/specifications.
+ removes `#include ".../ModelEntityItem.h"` from `Attribute.h`.
+ switches many operators to use ComponentItem or ReferenceItem
  instead of ModelEntityItem, and also switches some of these
  operators to use associations where they did not previously.
+ starts the convention of adding release notes to `doc/release/notes`.
+ fixes some tests to use the resource manager or operation manager
  to create objects because the way that reference items verify
  types often requires a resource manager to work properly.