Migrate clearcoat example to Feathers

[tevans-3]

Objective

• Migrates keyboard controls in examples/3d/clearcoat.rs to use vanilla Feathers widgets.
• Addresses one in a list of items in UI settings panel helpers for examples #25052.

Solution

• The previous clearcoat example used keyboard controls: by pressing a space key, a viewer of the example could toggle between "directional" and "point" light. This PR replaces that setup with a UI control pane which provides a total of 5 UI controls. In addition to two separate buttons for directional and point lights, there are new slider controls for illuminance and intensity and a scalar input for rotation speed. I found it kind of fun to make the spheres spin really quickly.

Testing

• I visually inspected and "end-user" tested the example several times. All behavior worked as I expected.
• Q: Are there any parts that need more testing? A: I don't think so.
• Reviewers can test these changes by running the following command from within bevy's root directory:

cargo run --example clearcoat ---features="pbr_multi_layer_material_textures" ---features="bevy_feathers"

Showcase

[IceSentry]

A few quick notes:

• There's too many nested containers. You don't need a background behind the thing that looks like it can fold but also the controls seem to be nested in a container with a different color which also seem unnecessary
• The light choice should use some kind of radio button that clearly indicates which is currently selected and doesn't let users select another one
• I haven't used bsn that much yet but I'm pretty sure you should be able to extract some things to functions instead of having just one giant bsn with dozens of level of identation.

[tevans-3]

@IceSentry

Thanks for the notes. I removed some containers, reduced the nesting, and extracted a bunch of the inlined logic into separate functions.

Here's what the pane looks like now: