Trait + types + registry for Teksilo's widget previewer infrastructure.

This crate is framework-side, UI-free: it defines the WidgetCatalog trait, the KnobSpec / KnobValues types, and the inventory-backed registry. The 3-pane previewer GUI lives in the sibling teksilo-preview-ui crate; per-application binaries (teksilo-widgets-previewer, etc.) link the two together along with their own widget set.

Authoring a catalog impl

use teksilo_preview::{
    register_widget_catalog, KnobSpec, KnobValues, PreviewVariant,
    KnobOverrides, WidgetCatalog,
};
use teksilo_core::widget::Widget;
use my_widgets::Button;

impl WidgetCatalog for Button {
    fn id() -> &'static str { "button" }
    fn group() -> &'static str { "Controls" }
    fn display_name() -> &'static str { "Button" }
    fn knobs() -> KnobSpec {
        KnobSpec::new()
            .text("label", "Label", "Click me")
            .bool_("disabled", "Disabled", false)
    }
    fn variants() -> Vec<PreviewVariant> {
        vec![
            PreviewVariant::defaults("default"),
            PreviewVariant::knobs("disabled",
                KnobOverrides::new().bool_("disabled", true)),
        ]
    }
    fn build(_variant: &str, knobs: &KnobValues) -> Box<dyn Widget> {
        let label = knobs.text("label").get();
        Box::new(Button::new(label).enabled(!knobs.bool_("disabled").get()))
    }
}
register_widget_catalog!(Button);