Add support for custom `enum` as sketch parameter

[abey79]

This PR adds support for custom enum to be used as sketch parameter, via the same Widget derive macro and #[sketch_widget] helper attribute.

The generated code distinguishes two cases: 1) "simple", C-style enums (no variant contains data) 2) "complex" enums (one or more variant has named or unnamed fields)

The former are displayed as a simple label + combo box that align to the UI grid. The latter offer a hierarchical UI that displays UI for nested data.

The ui_demo.rs example is updated to showcase the change.

#[sketch_widget]
#[derive(Default)]
enum SimpleEnum {
    #[default]
    Poodle,
    Corgy,
    Dalmatian,
}

#[sketch_widget]
#[derive(Default)]
enum CustomEnum {
    Variant1 {
        #[param(slider, min = 0.0, max = 1.0)]
        some_float: f64,

        #[skip]
        incompatible: IncompatibleStruct,
    },
    Variant2(
        bool,
        #[param(slider, min = 0.0, max = 1.0)] f64,
        #[skip] IncompatibleStruct,
    ),
    #[default]
    Variant3,
}

Limitations

• Using self to refer to another field in a #[param] attribute is not possible with enum fields.
• The whiskers-derive crate code is getting uglier and uglier.
• The UI for complex-enum empty variant is not very nice.