Application menus have been removed from gtk and gtkmm.
Use window menus (menubars).

* examples/book/buildapp: Remove all app_menu.ui and move the menu items
to gears_menu.ui. (Move from application menu to window menu.)
* examples/book/application: Rename app_and_win_menus to menubar.
* examples/book/menus/main_menu/exampleapplication.cc:
Use ApplicationWindow::set_show_menubar().