Allow Nuance items to put commands actions into the work UI

Summary:
Ref T12738. This doesn't actually do anything yet, but allows items to define commands that show up in the UI.

Adds a "Throw in Trash" item for complaints.

This construction will allow future changes to add an EngineExtension which can provide generic/default commands across item types.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12738

Differential Revision: https://secure.phabricator.com/D18009