Add Window::set_transparent

Provide a hint to system compositor whether the window is transparent or
not. Only implemented on macOS and Wayland for now.

src/platform_impl/orbital/window.rs

@@ -240,6 +240,9 @@ impl Window {
             .expect("failed to set title");
     }
 
+    #[inline]
+    pub fn set_transparent(&self, _transparent: bool) {}
+
     #[inline]
     pub fn set_visible(&self, _visibility: bool) {}