... in which we
+ Add an operations panel UI.
+ Add a new smtk::view::AvailableOperators class that tracks
  a selection and an operation manager, calling observers when
  the list of operations changes (some work TBD).
+ Add `{observe,unobserve}Metadata` to smtk::operation::Manager so
  that new observers can be immediately passed the list of extant
  operation metadata.
+ Respond to selections in modelbuilder.
+ Add a `primaryAssociation()` method to operation metadata
  that returns the smtk::attribute::ReferenceItem for each
  operation (or nullptr if the operation's parameters don't
  support association).
+ Fix logic in AvailableOperators to test whether each
  operator accepts **all** the selected items and that
  the **number** of selected items is appropriate.
+ Remove some debug printouts and use the available-ops observer
  to print when the list changes.
+ Add a new subsystem to SMTK: **workflow**.
  The workflow system will host a task-dag used to provi...