Add a slider to Paper Thickness preference for Onion Skin

[Snapai]

I'm always tweaking the paper thickness value, depending on what I'm trying to animate while using onion skinning.

In order to see whether my adjustments are appropriate, I have to click the input box to edit the value, then press enter (deselecting the box) to see the results.

Since it's a simple 0-100 value, could a slider be added to this, to make tweaking it a little easier, like the color values below it?

(It's probably arguable that some of these settings should be scene-specific not overall user preferences, in a different window/docker, but that seems like a much larger discussion than just adding a slider.)