Feature request: Colored Sliders and Radio Button

I'm trying to create colored sliders and radio buttons, similar to these ones:

Since egui doesn't offer an API for this right now, I had to copy the `egui::RadioButton` types into my project, and extending its functionality. But I'm hesitant to do the same for `egui::Slider`, as there is a big of chunk of code there. It would be amazing if coloring these widgets is supported by egui natively.

I'm happy to take a jab at adding these to egui, but I wanted to get some agreements on the API first. For RadioButtons, I propose that we can add arguments `base_color: Color32, checked_color: Color32` to `RadioButton::new()`. For sliders, I'd like for the API to support transitions, and its not obvious to me how the API should look like.

I'd appreciate feedback on
1. Should egui have an API for coloring sliders?
2. If yes, how can I help add one?
3. If no, what would be the recommended approach to implement this in my own project? Is there a better way other than copying `egui::Slider` and extending it?

Start by reading the existing egui::RadioButton and egui::Slider APIs and compare their current customization options. The issue needs maintainer agreement on whether colored sliders are supported and what the API should be; done would be a settled API direction and an agreed implementation path.

Written by the indexing model from the issue text.