Multiple mouse button support (#135)

Add support for primary, secondary and middle mouse buttons. Also improve ability to click things in low FPS situations.

This introduces a lot of breaking changes:

    Backends/integrations now pass mouse events via the even stream.
    Response has an interface of mostly methods instead of public members.
    input.mouse is now input.pointer and has new interface.


* Rename 'mouse' to 'pointer' everywhere (pointer = mouse or touch)

* Make Response::clicked and Response::double_clicked into methods

* Remove Response::active and add dragged() and interact_pointer_pos()

* Support multiple mouse buttons

* Make PointerState interface all methods

* Make most members of Response private

eframe/examples/hello_world.rs

@@ -29,7 +29,7 @@ impl epi::App for MyApp {
                 ui.text_edit_singleline(name);
             });
             ui.add(egui::Slider::u32(age, 0..=120).text("age"));
-            if ui.button("Click each year").clicked {
+            if ui.button("Click each year").clicked() {
                 *age += 1;
             }
             ui.label(format!("Hello '{}', age {}", name, age));