feat: add Window::is_visible (#2169)

* feat: add `Window::is_visible`

* use `Option<bool>`

* update doc

* move it right after `set_visible`

src/platform_impl/macos/window.rs

@@ -502,6 +502,12 @@ impl UnownedWindow {
         }
     }
 
+    #[inline]
+    pub fn is_visible(&self) -> Option<bool> {
+        let is_visible: BOOL = unsafe { msg_send![*self.ns_window, isVisible] };
+        Some(is_visible == YES)
+    }
+
     pub fn request_redraw(&self) {
         AppState::queue_redraw(RootWindowId(self.id()));
     }