Bug 1082444 - Handle additional events when FxA EntrySheet is displayed. r=ferjm

  * Specifically, listen for 'home', 'holdhome', and 'activityrequesting', and
    when those are caught, if the EntrySheet is open, close it.
  * Also, don't try to open the EntrySheet if navigator.onLine is false; show
    an error message instead.