Rich text for all widgets (#855)

Introduce `RichText` and `WidgetText`

egui_demo_lib/src/apps/http_app.rs

@@ -116,9 +116,9 @@ impl epi::App for HttpApp {
                     }
                     Err(error) => {
                         // This should only happen if the fetch API isn't available or something similar.
-                        ui.add(
-                            egui::Label::new(if error.is_empty() { "Error" } else { error })
-                                .text_color(egui::Color32::RED),
+                        ui.colored_label(
+                            egui::Color32::RED,
+                            if error.is_empty() { "Error" } else { error },
                         );
                     }
                 }