Add method to RichText making it easier to construct layout jobs (#3319)

* Slightly refactor and expose some parts of rich text to make it easier to construct layout jobs

* Cargo fmt

* Add documentation

crates/egui/src/widget_text.rs

@@ -269,12 +269,66 @@ impl RichText {
         fonts.row_height(&font_id)
     }
 
+    /// Append to an existing [`LayoutJob`]
+    ///
+    /// Note that the color of the [`RichText`] must be set, or may default to an undesirable color.
+    ///
+    /// ### Example
+    /// ```
+    /// use egui::{Style, RichText, text::LayoutJob, Color32, FontSelection, Align};
+    ///
+    /// let style = Style::default();
+    /// let mut layout_job = LayoutJob::default();
+    /// RichText::new("Normal")
+    ///     .color(style.visuals.text_color())
+    ///     .append_to(
+    ///         &mut layout_job,
+    ///         &style,
+    ///         FontSelection::Default,
+    ///         Align::Center,
+    ///     );
+    /// RichText::new("Large and underlined")
+    ///     .color(style.visuals.text_color())
+    ///     .size(20.0)
+    ///     .underline()
+    ///     .append_to(
+    ///         &mut layout_job,
+    ///         &style,
+    ///         FontSelection::Default,
+    ///         Align::Center,
+    ///     );
+    /// ```
+    pub fn append_to(
+        self,
+        layout_job: &mut LayoutJob,
+        style: &Style,
+        fallback_font: FontSelection,
+        default_valign: Align,
+    ) {
+        let (text, format) = self.into_text_and_format(style, fallback_font, default_valign);
+
+        layout_job.append(&text, 0.0, format);
+    }
+
     fn into_text_job(
         self,
         style: &Style,
         fallback_font: FontSelection,
         default_valign: Align,
     ) -> WidgetTextJob {
+        let job_has_color = self.get_text_color(&style.visuals).is_some();
+        let (text, text_format) = self.into_text_and_format(style, fallback_font, default_valign);
+
+        let job = LayoutJob::single_section(text, text_format);
+        WidgetTextJob { job, job_has_color }
+    }
+
+    fn into_text_and_format(
+        self,
+        style: &Style,
+        fallback_font: FontSelection,
+        default_valign: Align,
+    ) -> (String, crate::text::TextFormat) {
         let text_color = self.get_text_color(&style.visuals);
 
         let Self {
@@ -295,7 +349,6 @@ impl RichText {
             raised,
         } = self;
 
-        let job_has_color = text_color.is_some();
         let line_color = text_color.unwrap_or_else(|| style.visuals.text_color());
         let text_color = text_color.unwrap_or(crate::Color32::TEMPORARY_COLOR);
 
@@ -336,20 +389,20 @@ impl RichText {
             default_valign
         };
 
-        let text_format = crate::text::TextFormat {
-            font_id,
-            extra_letter_spacing,
-            line_height,
-            color: text_color,
-            background: background_color,
-            italics,
-            underline,
-            strikethrough,
-            valign,
-        };
-
-        let job = LayoutJob::single_section(text, text_format);
-        WidgetTextJob { job, job_has_color }
+        (
+            text,
+            crate::text::TextFormat {
+                font_id,
+                extra_letter_spacing,
+                line_height,
+                color: text_color,
+                background: background_color,
+                italics,
+                underline,
+                strikethrough,
+                valign,
+            },
+        )
     }
 
     fn get_text_color(&self, visuals: &Visuals) -> Option<Color32> {