UI specific color enum

## What problem does this solve or what need does it fill?

Bevy UI's visual components like `BackgroundColor`, `BorderColor`, `TextColor` and `ImageNode` only take a flat `Color` and there is nowhere to add support for other color effects.

Related #18139

## What solution would you like?

Replace the UI `Color` fields with a `UiColor` enum:
```rust
pub enum UiColor {
Color(Color),
/// Once gradients support is added
Gradient(Gradient),
}
```
`UiColor` would implement the `Color` conversion traits so that code like:
```rust
commands.spawn((Node::default(), BackgroundColor(ORANGE_200.into())));
```
would remain unchanged. And to draw a node with gradient you'd use:
```rust
commands.spawn((
Node::default(),
BackGroundColor(LinearGradient::to_left(vec![Color::WHITE.into(), ORANGE.into(), (BLUE_200.into(), Val::Px(50.)).into()]).into())
));
```

## What alternative(s) have you considered?

I don't see any viable alternatives.

## Additional context