LibGUI: Forget some of Window's widgets eagerly on widget unparenting

Previously the focused widget would only get cleared on replacement or
on destruction (being a WeakPtr and all.) This could lead to window
dispatching events to a focused widget after it had been removed from
the window's widget tree.

The same issue existed for the hovered widget, etc. So this patch
makes sure that we eagerly clear the various widget pointers in Window
immediately when they are removed from the window's widget tree.

Libraries/LibGUI/Widget.cpp

@@ -123,6 +123,8 @@ void Widget::child_event(Core::ChildEvent& event)
             else
                 invalidate_layout();
         }
+        if (event.child() && Core::is<Widget>(*event.child()))
+            window()->did_remove_widget({}, Core::to<Widget>(*event.child()));
         update();
     }
     return Core::Object::child_event(event);