修复控件树重建，修复hover状态不正确

example/src/main.cpp

@@ -16,18 +16,17 @@ int main(int argc, char* argv[]) {
 	window->show();
 
 	mirage_app::get_render_context()->setup_surface(window.get());
-
 	widget_manager::get().init_window(window);
 
 	auto weak_border = widget_manager::get().new_widget<mborder>(window.get());
-	auto border = weak_border.lock();
+
 	auto button = widget_manager::get().new_widget<mbutton>(window.get());
-	border->set_content(button.lock())
-	.margin({5});
+
+	auto border = weak_border.lock();
+	border->set_content(button.lock()).margin({ 5 });
 
 	window->set_content(border);
 
-
 	app.run();
 	return 0;
 }