workflows: widget->action name change

* Changes the naming of Holding Pen "widgets" to "actions" to
  more clearly distinguish these UI elements from widgets elsewhere
  in Invenio. The name "action" better suits these types of UI
  components.

* Renames several files to make use of this new naming strategy.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>