Update slider / scrollbar / number_input to use pointer capture

Widgets that listen for drag events should also capture the pointer when dragging.

(Note that this is not true for widgets that do true drag-and-drop, but we don't have any of those. This only applies to widgets that self-drag, like sliders).

This includes:

* Color Plane
* Color Wheel
* Slider
* Scrollbar
* FeathersNumberInput (which has its own dragging handlers since it's built around a text input)

...or just search for observers that listen for DragStart.

@alice-i-cecile @kfc35

Search for observers that listen for DragStart, then inspect the Color Plane, Color Wheel, Slider, Scrollbar, and FeathersNumberInput drag handlers. Done means self-dragging widgets capture the pointer while dragging, while true drag-and-drop behavior remains unchanged.

Written by the indexing model from the issue text.