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.

crates/egui_demo_lib/src/demo/context_menu.rs

@@ -80,6 +80,8 @@ impl super::View for ContextMenus {
         ui.label("Right-click plot to edit it!");
         ui.horizontal(|ui| {
             self.example_plot(ui).context_menu(|ui| {
+                ui.set_min_width(220.0);
+
                 ui.menu_button("Plot", |ui| {
                     if ui.radio_value(&mut self.plot, Plot::Sin, "Sin").clicked()
                         || ui
@@ -96,12 +98,12 @@ impl super::View for ContextMenus {
                     ui.add(
                         egui::DragValue::new(&mut self.width)
                             .speed(1.0)
-                            .prefix("Width:"),
+                            .prefix("Width: "),
                     );
                     ui.add(
                         egui::DragValue::new(&mut self.height)
                             .speed(1.0)
-                            .prefix("Height:"),
+                            .prefix("Height: "),
                     );
                     ui.end_row();
                     ui.checkbox(&mut self.show_axes[0], "x-Axis");

crates/egui_demo_lib/src/demo/tests.rs

@@ -486,6 +486,10 @@ fn response_summary(response: &egui::Response, show_hovers: bool) -> String {
         }
     }
 
+    if response.long_touched() {
+        writeln!(new_info, "Clicked with long-press").ok();
+    }
+
     new_info
 }