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

egui_demo_lib/src/apps/demo/sliders.rs

@@ -73,7 +73,7 @@ impl Sliders {
                 You can always see the full precision value by hovering the value.",
             );
 
-            if ui.button("Assign PI").clicked {
+            if ui.button("Assign PI").clicked() {
                 self.value = std::f64::consts::PI;
             }
         }