Add `ga` events for metrics

[cvan]

Need to run these by the Metrics folks, but these come to mind. (This is not a complete list; will adjust as needed.)

• adding scenes
• removing scenes
• entering VR
  • time since page was loaded
• exiting VR
  • time in experience
• VR headset events
  • connected
  • disconnected
  • mounted
  • dismounted
  • entered VR
  • exited VR
  • focussed (by the UA, OS, VR headset)
  • blurred (by the UA, OS, VR headset)
• app launch
  • time since idle, if return visit
  • time to first paint
  • time to interactive
  • client-side errors, if any
  • client-side warnings, if any
• scene launch
  • time to first paint
  • time to interactive
  • client-side errors, if any
  • client-side warnings, if any
• app exit
  • time spent