(WIP) Softkeys 1.0

Summary:
Redo how contextually based preferences are managed. Instead of being simply
         disabled, we dynamically add and remove them as appropriate based on selected
         UI mode. Added Softkey preferenece to InterfaceSettings but it does not exist
         yet. So don't click it ;P

Test Plan: nothing yet

Reviewers: synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D525