message: Add button enum comment
diff: <element name="SOURCE" since="4.5" />
  <element name="SHUFFLE" since="4.5" />
  <element name="REPEAT" since="4.5" />
  + <!-- Navigation Buttons -->
  <element name="NAV_CENTER_LOCATION" since="5.2" />
  <element name="NAV_ZOOM_IN" since="5.2" />
  <element name="NAV_ZOOM_OUT" since="5.2" />
  