+ Mark models as clean/dirty after each operator runs relative to
  the last time they were saved to a file.
  Models will always have a "clean" integer property defined
  that is 0 when the model is dirty and non-zero when it is clean.
  CMB uses this to determine whether there are unsaved models before
  requiring acknowledgment from the user for some actions.

+ Operators that save or load models (in SMTK-**native** JSON format,
  not session-specific filetypes) can indicate their outputs are
  "clean," which will cause the model property mentioned above to be
  set properly. All other operators mark modified and created
  entities (or their parent models) as dirty.

+ Add a new StoredResource class to the model namespace that holds
  information about session-specific model files (i.e., CAD files),
  auxiliary geometry files (e.g., images). Each file has a URL,
  a set of related model entities, and methods for indicating whether
  it is modified or not.  This change also includes additions to
  enums in Resource and ResourceSet since they previously only tracked
  attribute-related resources.

+ The model Manager class now has a `resources()` method that
  constructs or updates a ResourceSet containing StoredResource
  entries for files related to its model entities and returns
  the ResourceSet.