Remove nalgebra dependency

- Implement our own `Point` and `Vector` types
  - Make `Rectangle` not generic

src/widget/button.rs

@@ -263,7 +263,7 @@ pub trait Renderer {
     fn draw(
         &mut self,
         cursor_position: Point,
-        bounds: Rectangle<f32>,
+        bounds: Rectangle,
         state: &State,
         label: &str,
         class: Class,