Add builder methods to bevy_ui's Style struct

[nicoburns]

Objective

Make it more ergonomic to create UI nodes

Solution

• Builder methods have been added to the Style object
• From impls have been added to Val (f32 and i32 both map to Val::Px)

Notes

I've added the methods to Style, but I'm they might be better on NodeBundle? Advantage of that is that it cuts out an extra level of nesting. Disadvantage is that we'd presumably have to add them for every bundle.

---

Changelog

• Added builder methods to bevy_ui::Style

[ickshonpe]

I think the builder methods would be more useful on the node bundles probably, or they could be implemented for both the bundles and Style.

[alice-i-cecile]

Perhaps we can use a trait here with default impls to reduce boilerplate in the engine?

[nicoburns]

Perhaps we can use a trait here with default impls to reduce boilerplate in the engine?

Hmm... I suppose you could probably set it up so that the only method each implementor of the trait needed to implement was something like:

fn apply_style(mut self, applicator: FnMut(&mut Style)) -> Self {
    applicator(&mut self.style);
    self
}

It would leak the implementation detail through to the docs though (the methods for layout would all be grouped together on the trait separately from other methods)

[alice-i-cecile]

Leaking that through to the docs seems net good to me? Being able to see that structure is good, and as a user I'd really like to be able to easily use whatever ergonomics solution we end up on with my own bundle types.