[Gradient Slider] Gradient Stop Collection does not respond to external add/removals

The `GradientStop` has an event for the color updating and the offset updating, and the `GradientSliderThumb` will use them to update.

However, the `GradientStopCollection` does not appear to have any event for a stop being added or removed, so the `GradientSlider` is not subscribed to any event which handles the adding or removing of a stop.