Add method to RichText making it easier to construct layout jobs

[OmegaJak]

Closes #3318

This refactors the existing code that was used inside RichText.into_text_job() to generate a LayoutJob to also expose a method that appends to a LayoutJob passed in from the outside. This makes it possible to construct a LayoutJob using only RichTexts, without having to manually touch TextFormat.

In my code I added an extension method to UI (which didn't seem fitting for core egui) that looks like this:

fn create_default_layout_job(&self, rich_texts: Vec<RichText>) -> LayoutJob {
    let mut layout_job = LayoutJob::default();
    for rich_text in rich_texts.into_iter() {
        rich_text.append_to(
            &mut layout_job,
            self.style(),
            egui::FontSelection::Default,
            egui::Align::Center,
        )
    }

    layout_job
}

which can be used like so:

let layout_job = ui.create_default_layout_job(vec![
    RichText::new("Using: ").color(ui.style().visuals.text_color()),
    RichText::new(format!("{} ", self.current_window.app_name)).strong(),
    RichText::new(format!("(started using {})", time_using_current))
        .color(ui.style().visuals.text_color()),
]);

This, in my opinion, is a much more ergonomic API that was available previously. Compare the above code to the code inside the linked #3318.

Note that the color of text must be specified in the RichText since WidgetText.into_text_job() always sets WidgetTextJob::job_has_color to true when converting from a LayoutJob, or the text will show up as green.

I'm not sure if this change is appropriate for core egui, but since it helped me, I thought I'd share and see.

[emilk]

Nice!