On touch screens, press-and-hold equals a secondary click (#4195)

* Closes https://github.com/emilk/egui/issues/3444
* Closes https://github.com/emilk/egui/issues/865

On a touch screen, if you press down on a widget and hold for 0.6
seconds (`MAX_CLICK_DURATION`), it will now trigger a secondary click,
i.e. `Response::secondary_clicked` will be `true`. This means you can
now open context menus on touch screens.

examples/custom_keypad/src/keypad.rs

@@ -174,7 +174,7 @@ impl Keypad {
     pub fn show(&self, ctx: &egui::Context) {
         let (focus, mut state) = ctx.memory(|m| {
             (
-                m.focus(),
+                m.focused(),
                 m.data.get_temp::<State>(self.id).unwrap_or_default(),
             )
         });