Rename DisplayUnit members to align them with the platform style conventions and with implementations

* remove is-prefixes
* inverse the logic of internal/external
* rename dpiX/dpiY into deviceXDPI/deviceYDPI to align with:
  <http://msdn.microsoft.com/en-us/library/ie/ms535868(v=vs.85).aspx>
  (not part of any spec yet)

@hmin to review.