Added winit accessor

I originally removed this API to ensure no Cushy widgets were using
winit directly, but I've reintroduced it so that third party widgets can
still interact with winit if needed.

src/animation.rs

@@ -428,9 +428,6 @@ where
     }
 }
 
-/// A [`Animate`] implementor that has been boxed as a trait object.
-pub struct BoxedAnimation(Box<dyn Animate>);
-
 /// An animation that can be spawned.
 pub trait Spawn {
     /// Spawns the animation, returning a handle that tracks the animation.

src/window.rs

@@ -245,6 +245,9 @@ pub trait PlatformWindow {
     fn set_min_inner_size(&self, min_size: Option<Size<UPx>>);
     /// Sets the window's maximum inner size.
     fn set_max_inner_size(&self, max_size: Option<Size<UPx>>);
+
+    /// Returns a handle to the underlying winit window, if available.
+    fn winit(&self) -> Option<&winit::window::Window>;
 }
 
 /// A currently running Cushy window.
@@ -436,6 +439,10 @@ where
     fn set_ime_location(&self, location: Rect<Px>) {
         self.window.set_ime_location(location);
     }
+
+    fn winit(&self) -> Option<&winit::window::Window> {
+        self.window.winit()
+    }
 }
 
 /// The attributes of a Cushy window.