feat: add Window::is_decorated (#2172)

* feat: add `Window::is_decorated`

* move it right after `set_decorations`

src/platform_impl/windows/window.rs

@@ -544,6 +544,12 @@ impl Window {
         });
     }
 
+    #[inline]
+    pub fn is_decorated(&self) -> bool {
+        let window_state = self.window_state.lock();
+        window_state.window_flags.contains(WindowFlags::DECORATIONS)
+    }
+
     #[inline]
     pub fn set_always_on_top(&self, always_on_top: bool) {
         let window = self.window.clone();