Allow Nuance items to put commands actions into the work UI
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.
Reviewed By: chad
Maniphest Tasks: T12738
Differential Revision: https://secure.phabricator.com/D18009