Make each `smtk::project::Project` own a `task::Manager`.

Before, the task::Manager was presumed to be application state. But because we only want to show tasks for a single active document (i.e., project) at a time, move it into project. This also helps decide where the task state should live.

doc/release/notes/operation-hints.rst

+Operation hint for switching the active task
+--------------------------------------------
+
+Any operation can now request the active task be switched
+by providing a hint on its result attribute.
+Use the ``smtk::operation::addActivateTaskHint()`` function
+to add the hint before your operation completes.
+Then, the :smtk:`pqSMTKOperationHintsBehavior` object will
+observe when the operation has completed and process the
+hint and attempt to switch to the matching task.
+See ``smtk::project::Read`` for an example.