Update the menus/main_menu and menus_and_toolbars examples

* docs/tutorial/C/figures/main_menu.png:
* docs/tutorial/C/figures/menus_and_toolbars.png: Updated pictures.
* examples/book/menus/main_menu/examplewindow.[h|cc]: Use Gtk::Builder to
create the toolbar.
* examples/book/menus_and_toolbars/examplewindow.[h|cc]: Use Gtk::Builder to
create the toolbar. Add the Rain tool button, which was deleted when
Gtk::UIManager was abandoned. Use Gtk::Application::set_accel_for_action().
* examples/book/menus_and_toolbars/main.cc: Add the app to ExampleWindow's
constructor.