Implement the :focus pseudo-class selector

Fixes #5460. This supports for simple focusable elements that are their own
DOM anchors, like text `input` fields.

components/layout/wrapper.rs

@@ -581,6 +581,14 @@ impl<'le> TElement<'le> for LayoutElement<'le> {
         }
     }
 
+    #[inline]
+    fn get_focus_state(self) -> bool {
+        unsafe {
+            let node: &Node = NodeCast::from_actual(self.element);
+            node.get_focus_state_for_layout()
+        }
+    }
+
     #[inline]
     fn get_id(self) -> Option<Atom> {
         unsafe {