Adds a setLabel function to ActionView

Summary: I copied some JavaScript. I think I did OK. Though, I couldn't stop icons from writing HTML as undefined. Anyways, this give us more options, like labels, when drawing ActionViews. I'll update the UI in other places like Policy too.

Test Plan:
Built and tested on the search ui dropdown.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12522