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/font_book.rs

@@ -31,7 +31,7 @@ impl FontBook {
                     ui.label(format!("{}\nU+{:X}\n\nClick to copy", name, chr as u32));
                 };
 
-                if ui.add(button).on_hover_ui(tooltip_ui).clicked {
+                if ui.add(button).on_hover_ui(tooltip_ui).clicked() {
                     ui.output().copied_text = chr.to_string();
                 }
             }
@@ -81,7 +81,7 @@ impl super::View for FontBook {
             ui.label("Filter:");
             ui.text_edit_singleline(&mut self.filter);
             self.filter = self.filter.to_lowercase();
-            if ui.button("ｘ").clicked {
+            if ui.button("ｘ").clicked() {
                 self.filter.clear();
             }
         });