Update SCTK to 0.18.0

The update is pretty minor, however we support now
`WindowEvent::Occluded` when xdg-shell v6 is available.

It also adds support for `Window::show_window_menu`.

Fixes #2927.

src/platform_impl/linux/wayland/window/mod.rs

@@ -374,6 +374,13 @@ impl Window {
         None
     }
 
+    #[inline]
+    pub fn show_window_menu(&self, position: Position) {
+        let scale_factor = self.scale_factor();
+        let position = position.to_logical(scale_factor);
+        self.window_state.lock().unwrap().show_window_menu(position);
+    }
+
     #[inline]
     pub fn drag_resize_window(&self, direction: ResizeDirection) -> Result<(), ExternalError> {
         self.window_state