Make rate applying more consistent and give more information

  * If only one image is seen, any rating done by using the lower panel or
  keyboard should apply to it, whatsoever the current selection is.
  * If the key accels are used to rate any image that is selected,
  the rate should be applied to all the selection.

In general, all the ratings done should show a toast message saying that the
rating has been applied, and the affected images.