(#52) portal, testing: The page controller now handles visibility events to coordinate the visibility of widget controllers.
      The portal now provides the AngularJS axVisibilityService to inform directives about changes to their visibility.
      The portal also now provides the axHeartbeat service to register callbacks to run right after the next event-bus tick plus $apply.
Merge branch 'feature/52-manage-visibility-events--rebase-onto-59'

* feature/52-manage-visibility-events--rebase-onto-59:
  (#52) portal, testing: The page controller now handles visibility events to coordinate the visibility of widget controllers.
        The portal now provides the AngularJS axVisibilityService to inform directives about changes to their visibility.
        The portal also now provides the axHeartbeat service to register callbacks to run right after the next event-bus tick plus $apply.