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

emath/src/vec2.rs

@@ -21,6 +21,9 @@ pub const fn vec2(x: f32, y: f32) -> Vec2 {
     Vec2 { x, y }
 }
 
+// ----------------------------------------------------------------------------
+// Compatibility and convenience conversions to and from [f32; 2]:
+
 impl From<[f32; 2]> for Vec2 {
     fn from(v: [f32; 2]) -> Self {
         Self { x: v[0], y: v[1] }
@@ -33,6 +36,47 @@ impl From<&[f32; 2]> for Vec2 {
     }
 }
 
+impl From<Vec2> for [f32; 2] {
+    fn from(v: Vec2) -> Self {
+        [v.x, v.y]
+    }
+}
+
+impl From<&Vec2> for [f32; 2] {
+    fn from(v: &Vec2) -> Self {
+        [v.x, v.y]
+    }
+}
+
+// ----------------------------------------------------------------------------
+// Compatibility and convenience conversions to and from (f32, f32):
+
+impl From<(f32, f32)> for Vec2 {
+    fn from(v: (f32, f32)) -> Self {
+        Self { x: v.0, y: v.1 }
+    }
+}
+
+impl From<&(f32, f32)> for Vec2 {
+    fn from(v: &(f32, f32)) -> Self {
+        Self { x: v.0, y: v.1 }
+    }
+}
+
+impl From<Vec2> for (f32, f32) {
+    fn from(v: Vec2) -> Self {
+        (v.x, v.y)
+    }
+}
+
+impl From<&Vec2> for (f32, f32) {
+    fn from(v: &Vec2) -> Self {
+        (v.x, v.y)
+    }
+}
+
+// ----------------------------------------------------------------------------
+
 impl Vec2 {
     pub const X: Vec2 = Vec2 { x: 1.0, y: 0.0 };
     pub const Y: Vec2 = Vec2 { x: 0.0, y: 1.0 };