Adding context menus.

The context menu handler now defers to CEF.java to see whether a context menu
has been provided by the browser. If the browser has set a context menu
that is displayed instead of the CEF one.

Caveats:
* I am defering to CEF.java using syncExec so that I can get find out
  whether the Browser has a menu then use this information back in the
  handler. I am doing this because the documentation explicitly requests
  that I do not hold a reference to the menu model or menu parameters
  outside of the handler method.