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/widgets.rs

@@ -97,7 +97,7 @@ impl Widgets {
             if ui
                 .add(Button::new("Click me").enabled(self.button_enabled))
                 .on_hover_text("This will just increase a counter.")
-                .clicked
+                .clicked()
             {
                 self.count += 1;
             }
@@ -146,7 +146,7 @@ impl Widgets {
         ui.horizontal(|ui| {
             ui.label("Single line text input:");
             let response = ui.text_edit_singleline(&mut self.single_line_text_input);
-            if response.lost_kb_focus {
+            if response.lost_kb_focus() {
                 // The user pressed enter.
             }
         });