Main menu example: Use Gtk::Application to display an app menu

* examples/Makefile.am: Add main_menu/exampleapplication.[h|cc].
* examples/book/menus/main_menu/exampleapplication.[h|cc]: New files.
* examples/book/menus/main_menu/examplewindow.[h|cc]:
* examples/book/menus/main_menu/main.cc: Derive from Gtk::Application and
Gtk::ApplicationWindow, similar to the application/app_and_win_menus example.

Thanks to Jürgen Kleber, who attached similar code to
https://mail.gnome.org/archives/gtkmm-list/2014-November/msg00012.html
Menus and Toolbars chapter: Partial update

* docs/tutorial/C/index-in.docbook: Note that some of the described classes
have been deprecated in gtk+. Add links to the menus_and_toolbars example.