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_web/src/backend.rs

@@ -81,6 +81,9 @@ pub struct WebInput {
     /// Is this a touch screen? If so, we ignore mouse events.
     pub is_touch: bool,
 
+    /// Required because we don't get a position on touched
+    pub latest_touch_pos: Option<egui::Pos2>,
+
     pub raw: egui::RawInput,
 }