Application example: Show the new command-line parsing API

* examples/Makefile.am: Delete exampleoptiongroup.[cc|h].
* examples/book/application/command_line_handling/exampleapplication.[cc|h]:
Show how to use Gio::Application::add_main_option_entry().
* examples/book/application/command_line_handling/exampleoptiongroup.[cc|h]:
Deleted files. Bug #727822.