Slider aesthetic improvement

The Hacknus slider https://github.com/hacknus/egui_double_slider/ is very cool, however there are aesthetic differences with the official egui slider that make mixing and matching the sliders less than perfect (Hacknus is the top slider).

Both sliders are set to width = 300, however they display different lengths. Note also the different thickness slider bar and rounded ends. Perhaps the blue colors are also different?

Btw, the Hacknus slider allows width setting as part of the builder pattern config for the slider while the native egui slider requires a hack:

```ui.spacing_mut().slider_width = 300.0; // Global value for slider width / native widget hack```

Lastly, the native slider has a DragValue box on the RHS of the slider, which the Hacknus version does not have. Using the new Atom layout tools described here https://github.com/emilk/egui/releases it should be possible easily to add text, DragValues, checkboxes etc. to the widget.

I will cross-post this as an egui issue suggestion.