Fix EditorSpinSlider grabber TextureRect doesn't follow parent when scrolling.

[WhalesState]

• Fixes: Focus "outline" for sliders moves with scrolling  #97825

[SaracenOne]

Hmm, the fix seems mostly fine, but I'm a little concerned about hardcoding the z-value to 1. While I don't think the z-value would change for an EditorSpinSlider, wouldn't it still make more sense to make it something like grabber->set_z_index(get_z_index() + 1); so it's always relative?

[SaracenOne]

My previous feedback was mistaken since all CanvasItems have z_relative set on by default, so this should be fine to merge as is after all 👍

[Mickeon]

This is relatively simple and easy to salvage, too.

[Repiteo]

Thanks!