Since bgo#757828 introduced g_application_set_resource_base_path,
we don't need to use a GtkBuilder to load the appmenu.

GtkApplication automatically loads menus from the GtkBuilder resource
located at "gtk/menus.ui", relative to the application's resource
base path (see g_application_set_resource_base_path ()). The menu
with the ID "app-menu" is taken as the application's app menu.

https://bugzilla.gnome.org/show_bug.cgi?id=757937