Add a Gtk::ListBox example

* examples/Makefile.am: Add book/listbox/example.
* examples/book/listbox/examplewindow.[h|cc]:
* examples/book/listbox/examplerow.[h|cc]:
* examples/book/listbox/main.cc: New files.
This example is a translation (with very small changes) to C++ of
gtk+/tests/testlist.c. Bug #708115.