Ui::add_sized does not work as expected with Slider

Ui::add_sized does not work as one would expect when Slider is passed in. Instead Spacing::slider_width is used for its width.

The expectation here is that slider can be resized by supplying size in add_sized.

Start by tracing Ui::add_sized when a Slider is passed in, then inspect where Spacing::slider_width supplies the slider width. The change is complete when the size supplied to add_sized controls the slider width instead of Spacing::slider_width.

Written by the indexing model from the issue text.