feat: add Window::is_decorated (#2172)

* feat: add `Window::is_decorated`

* move it right after `set_decorations`

src/platform_impl/macos/window.rs

@@ -1003,6 +1003,11 @@ impl UnownedWindow {
         }
     }
 
+    #[inline]
+    pub fn is_decorated(&self) -> bool {
+        self.decorations.load(Ordering::Acquire)
+    }
+
     #[inline]
     pub fn set_always_on_top(&self, always_on_top: bool) {
         let level = if always_on_top {