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/linux/x11/window.rs

@@ -984,6 +984,11 @@ impl UnownedWindow {
         }
     }
 
+    #[inline]
+    pub fn is_visible(&self) -> Option<bool> {
+        None
+    }
+
     fn update_cached_frame_extents(&self) {
         let extents = self
             .xconn