Fix infinite loop at HighlightedBox.

- It looks like using now deleted `changing_style` atribute
  to avoid infinite calls to `OnSyleSet` method doesn't work in
  GTK3 —until now, F-Spot/gtk3 was using an entire core to output
  the infinite warning «Gdk-WARNING **: gdk-frame-clock:
  layout continuously requested, giving up after 4 tries». Fixed
  implementing the desired behavior at `OnDrawn` method —also
  because `OnSyleSet` is deprecated now.