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.

Note that this commit removes constructors from subclasses
of `smtk::task::Task` since they need to call a deprecated
parent constructor.

This commit also removes signatures from `smtk::task::Instances`
since that also causes deprecation warnings (and potentially
further failures since the task-manager is required).

This also provides `Instances::createFromName()` signatures
that do not require a task manager (by recording the task manager
which owns Instances at construction time).