Add AtomLayout, abstracing layouting within widgets (#5830)

Today each widget does its own custom layout, which has some drawbacks:
- not very flexible
- you can add an `Image` to `Button` but it will always be shown on the
left side
  - you can't add a `Image` to a e.g. a `SelectableLabel`
- a lot of duplicated code

This PR introduces `Atoms` and `AtomLayout` which abstracts over "widget
content" and layout within widgets, so it'd be possible to add images /
text / custom rendering (for e.g. the checkbox) to any widget.

A simple custom button implementation is now as easy as this:
```rs
pub struct ALButton<'a> {
    al: AtomicLayout<'a>,
}

impl<'a> ALButton<'a> {
    pub fn new(content: impl IntoAtomics) -> Self {
        Self { al: content.into_atomics() }
    }
}

impl<'a> Widget for ALButton<'a> {
    fn ui(mut self, ui: &mut Ui) -> Response {
        let response = ui.ctx().read_response(ui.next_auto_id());

        let visuals = response.map_or(&ui.style().visuals.widgets.inactive, |response| {
            ui.style().interact(&response)
        });

        self.al.frame = self
            .al
            .frame
            .inner_margin(ui.style().spacing.button_padding)
            .fill(visuals.bg_fill)
            .stroke(visuals.bg_stroke)
            .corner_radius(visuals.corner_radius);

        self.al.show(ui)
    }
}

```

The initial implementation only does very basic layout, just enough to
be able to implement most current egui widgets, so:
- only horizontal layout
- everything is centered
- a single item may grow/shrink based on the available space
- everything can be contained in a Frame


There is a trait `IntoAtoms` that conveniently allows you to construct
`Atoms` from a tuple
```
   ui.button((Image::new("image.png"), "Click me!"))
```
to get a button with image and text.


This PR reimplements three egui widgets based on the new AtomLayout:
 - Button
   - matches the old button pixel-by-pixel
- Button with image is now [properly
aligned](https://github.com/emilk/egui/pull/5830/files#diff-962ce2c68ab50724b01c6b64c683c4067edd9b79fcdcb39a6071021e33ebe772)
in justified layouts
   - selected button style now matches SelecatbleLabel look
- For some reason the DragValue text seems shifted by a pixel almost
everywhere, but I think it's more centered now, yay?
 - Checkbox
- basically pixel-perfect but apparently the check mesh is very slightly
different so I had to update the snapshot
   - somehow needs a bit more space in some snapshot tests?
 - RadioButton
   - pixel-perfect
   - somehow needs a bit more space in some snapshot tests?

I plan on updating TextEdit based on AtomLayout in a separate PR (so
you could use it to add a icon within the textedit frame).

crates/egui_demo_lib/src/demo/widget_gallery.rs

@@ -322,7 +322,10 @@ mod tests {
         let mut harness = Harness::builder()
             .with_pixels_per_point(2.0)
             .with_size(Vec2::new(380.0, 550.0))
-            .build_ui(|ui| demo.ui(ui));
+            .build_ui(|ui| {
+                egui_extras::install_image_loaders(ui.ctx());
+                demo.ui(ui);
+            });
 
         harness.fit_contents();
 

crates/egui_demo_lib/tests/snapshots/demos/Grid Test.png

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