Major module reorganization:

Major module reorganization:
- new module TermViewer: holds widgets to render and interact with sequents
  and proofs at the CIC level
- new module ApplyStylesheets: functions to apply stylesheets and map
  terms and sequents to MathML Presentation
- new module InvokeTactics: it is a functor that gives back a module that
  defines one callback function for every tactic