Add with_child to simplify spawning when there will only be one child

[rparrett]

Objective

This idea came up in the context of a hypothetical "text sections as entities" where text sections are children of a text bundle.

commands
    .spawn(TextBundle::default())
    .with_children(|parent| {
        parent.spawn(TextSection::from("Hello"));
    });

This is a bit cumbersome (but powerful and probably the way things are headed). bsn! will eventually make this nicer, but in the mean time, this might improve ergonomics for the common case where there is only one TextSection.

Solution

Add a with_child method to the BuildChildren trait that spawns a single bundle and adds it as a child to the entity.

commands
    .spawn(TextBundle::default())
    .with_child(TextSection::from("Hello"));

Testing

I added some tests, and modified the button example to use the new method.

If any potential co-authors want to improve the tests, that would be great.

Alternatives

• Some sort of macro. See https://github.com/tigregalis/bevy_spans_ent/blob/main/examples/macro.rs#L20. I don't love this, personally, and it would probably be obsoleted by bsn!.
• Wait for bsn!
• Add with_children_batch that takes an Into<Iterator> of bundles.

  with_children_batch(vec![TextSection::from("Hello")])

  This is maybe not as useful as it sounds -- it only works with homogeneous bundles, so no marker components or styles.
• If this doesn't seem valuable, doing nothing is cool with me.

[alice-i-cecile]

Good: nice and simple but should make simple cases easier. The longer we can shield new users from closures the better.

[TimJentzsch]

A small change, but can make the code a lot easier to read.
Quite a common pattern in UI, but can also be useful in other contexts.

I like it!

[Olle-Lukowski]

This is simply a nice-to-have, those are always welcome IMO.