BackgroundColor support

[Sjael]

I's pretty common to need the ability to fade UI elements/images on and off, so I added a lens for BackgroundColor.

Simply tween the alpha value for two Color::rgb().set_a()

let selected_color = Color::RED;
let tween_opac = Tween::new(
    EaseFunction::QuadraticIn,
    Duration::from_secs(1),
    UiBackgroundColorLens {
        start: selected_color.set_a(0.9),
        end: selected_color.set_a(0.0),
    },
);
let animator = Animator::new(tween_opac);

It's very nice for flashing something on screen and removing after a delay, like a killfeed notification.

let selected_color = Color::RED;
let tween_opac_in = Tween::new(
    EaseFunction::QuadraticIn,
    Duration::from_millis(500),
    UiBackgroundColorLens {
        start: selected_color.set_a(0.0),
        end: selected_color.set_a(0.9),
    },
);
let tween_opac_out = Tween::new(
    EaseFunction::QuadraticIn,
    Duration::from_secs(1),
    UiBackgroundColorLens {
        start: selected_color.set_a(0.9),
        end: selected_color.set_a(0.0),
    },
).with_completed_event(
    TweenEvents::KillNotifEnded as u64,
);
let animator =  Animator::new(tween_opac_in
    .then(Delay::new(
        Duration::from_secs(15),
    ))
    .then(tween_opac_out)
);

This would also work for going between any 2 colors, but I think the opacity will be most used.

[djeedai]

Hi, thanks for the change!

I'm generally against adding more lenses, as really the system is designed for users to write their own, and I don't want to have to maintain every single use case. That being said, here I appreciate how common this is, and is in a sense an unfortunate regression of Bevy newtyping all its UI type a few releases ago. So OK for this change.

[djeedai]

Can you please update the CHANGELOG, and ideally add a small test for that new lens? Thanks!

[djeedai]

Need CHANGELOG update

[djeedai]

I added the CHANGELOG change myself just to complete the PR as we're nearing the Bevy 0.12 release and I want to be ready :) Thanks!