gpui: Improve Fill type

[huacnlee]

Release Notes:

• N/A

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I was changed Fill to use Background since PR #20812.

But it's not inconvenient to retrieve the background color already set in the current style.

For example: I have use StyleRefinement a lot in GPUI Component, I may define a style for an element and impl Styled for MyElement for allows user to override the element default style.

In this case, I often need to get the bg of user sets to do something, e.g.: Adjust that color opacity to border use.

struct MyElement {
    style: StyleRefinement,
}

impl Styled for MyElement {
    fn style(&mut self) -> &mut StyleRefinement {
        &mut self.style
    }
}

fn render(...) {
    let bg_color = self.style.background;
    // Adjust bg_color ...

    div()
        .bg(default_color)
        .border_color(...)
        .map(|this| this.style().refine(&self.style))
}

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So, I make a little change for Fill to split Solid and Background.

• Solid: Mostly if we use bg(gpui::red()) the Solid will be used.
• Background: For gradient background case.

And with color method to get the color from Fill.