Decouple BackgroundColor from UiImage (bevyengine#11165)

# Objective

Fixes bevyengine#11157.

## Solution

Stop using `BackgroundColor` as a color tint for `UiImage`. Add a
`UiImage::color` field for color tint instead. Allow a UI node to
simultaneously include a solid-color background and an image, with the
image rendered on top of the background (this is already how it works
for e.g. text).


![2024-02-29_1709239666_563x520](https://github.com/bevyengine/bevy/assets/12173779/ec50c9ef-4c7f-4ab8-a457-d086ce5b3425)

---

## Changelog

- The `BackgroundColor` component now renders a solid-color background
behind `UiImage` instead of tinting its color.
- Removed `BackgroundColor` from `ImageBundle`, `AtlasImageBundle`, and
`ButtonBundle`.
- Added `UiImage::color`.
- Expanded `RenderUiSystem` variants.
- Renamed `bevy_ui::extract_text_uinodes` to `extract_uinodes_text` for
consistency.

## Migration Guide

- `BackgroundColor` no longer tints the color of UI images. Use
`UiImage::color` for that instead.
- For solid color buttons, replace `ButtonBundle { background_color:
my_color.into(), ... }` with `ButtonBundle { image:
UiImage::default().with_color(my_color), ... }`, and update button
interaction systems to use `UiImage::color` instead of `BackgroundColor`
as well.
- `bevy_ui::RenderUiSystem::ExtractNode` has been split into
`ExtractBackgrounds`, `ExtractImages`, `ExtractBorders`, and
`ExtractText`.
- `bevy_ui::extract_uinodes` has been split into
`bevy_ui::extract_uinode_background_colors` and
`bevy_ui::extract_uinode_images`.
- `bevy_ui::extract_text_uinodes` has been renamed to
`extract_uinode_text`.

crates/bevy_ui/src/node_bundles.rs

@@ -93,10 +93,6 @@ pub struct ImageBundle {
     pub style: Style,
     /// The calculated size based on the given image
     pub calculated_size: ContentSize,
-    /// The background color, which serves as a "fill" for this node
-    ///
-    /// Combines with `UiImage` to tint the provided image.
-    pub background_color: BackgroundColor,
     /// The image of the node
     pub image: UiImage,
     /// The size of the image in pixels
@@ -140,10 +136,6 @@ pub struct AtlasImageBundle {
     pub style: Style,
     /// The calculated size based on the given image
     pub calculated_size: ContentSize,
-    /// The background color, which serves as a "fill" for this node
-    ///
-    /// Combines with `UiImage` to tint the provided image.
-    pub background_color: BackgroundColor,
     /// The image of the node
     pub image: UiImage,
     /// A handle to the texture atlas to use for this Ui Node
@@ -319,10 +311,6 @@ pub struct ButtonBundle {
     pub interaction: Interaction,
     /// Whether this node should block interaction with lower nodes
     pub focus_policy: FocusPolicy,
-    /// The background color, which serves as a "fill" for this node
-    ///
-    /// When combined with `UiImage`, tints the provided image.
-    pub background_color: BackgroundColor,
     /// The color of the Node's border
     pub border_color: BorderColor,
     /// The image of the node
@@ -349,13 +337,12 @@ pub struct ButtonBundle {
 impl Default for ButtonBundle {
     fn default() -> Self {
         Self {
-            focus_policy: FocusPolicy::Block,
             node: Default::default(),
             button: Default::default(),
             style: Default::default(),
-            border_color: BorderColor(Color::NONE),
             interaction: Default::default(),
-            background_color: Default::default(),
+            focus_policy: FocusPolicy::Block,
+            border_color: BorderColor(Color::NONE),
             image: Default::default(),
             transform: Default::default(),
             global_transform: Default::default(),