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/ios/window.rs

@@ -60,6 +60,11 @@ impl Inner {
         }
     }
 
+    pub fn is_visible(&self) -> Option<bool> {
+        warn!("`Window::is_visible` is ignored on iOS");
+        None
+    }
+
     pub fn request_redraw(&self) {
         unsafe {
             if self.gl_or_metal_backed {