Improve Menu API to facilitate external control

examples/game_of_life/src/main.rs

@@ -788,7 +788,7 @@ struct Controls {
     next_button: button::State,
     clear_button: button::State,
     speed_slider: slider::State,
-    preset_list: pick_list::State,
+    preset_list: pick_list::State<Preset>,
 }
 
 impl Controls {

examples/pick_list/src/main.rs

@@ -10,7 +10,7 @@ pub fn main() {
 #[derive(Default)]
 struct Example {
     scroll: scrollable::State,
-    pick_list: pick_list::State,
+    pick_list: pick_list::State<Language>,
     selected_language: Language,
 }