[demo] Link to the egui docs from the widget gallery

egui_demo_lib/src/apps/demo/toggle_switch.rs

@@ -92,3 +92,7 @@ fn toggle_ui_compact(ui: &mut egui::Ui, on: &mut bool) -> egui::Response {
 pub fn toggle(on: &mut bool) -> impl egui::Widget + '_ {
     move |ui: &mut egui::Ui| toggle_ui(ui, on)
 }
+
+pub fn url_to_file_source_code() -> String {
+    format!("https://github.com/emilk/egui/blob/master/{}", file!())
+}