Spin off the UI style and components into separate crates

[nu11ptr]

Is your feature request related to a problem? Please describe.
The UI for this project is very nice. It would be nice to be able to use it without reverse engineering in other projects

Describe the solution you'd like
Spin off UI styles and components into their own set of crates with very basic instructions how we can use both style and the custom widgets in our own projects (specific README or website docs covering usage)

Describe alternatives you've considered
Reverse engineering starting with the re_ui crate

Additional context
The style and custom widgets/components are richer and more featureful than those in stock egui. Either incorporate these into the main project or spin them off into an easily consumable format for other projects.

[emilk]

We're planning on making egui a lot easier to style with:

• CSS-like styling emilk/egui#3284

The goals is for re_ui to become a lot simpler as a consequence, requiring less "reverse engineering".