Added a type `Breadth`, that is similar to `Val` but with only evaluatable variants.
        Gave UiRect a type parameter, so it can take `Breadth` or `Val` values.
        Added tests for `Breadth` and `UiRect`.
        Changed style properties to use `Breadth` instead of `Val` for the padding and border properties.
        Changed `bevy_ui::flex::convert::from_rect` to take an `UiRect<T: Into<Val>>` instead of a `UiRect`.
        Made minimal necessary changes to the examples.

…padding and border values in `bevy_ui::flex::convert::from_style`