Clipboard and Socket examples: Make them non-unique.

* examples/book/clipboard/ideal/main.cc:
* examples/book/clipboard/simple/main.cc: Add Gio::APPLICATION_NON_UNIQUE to
the calls to Gtk::Application::create(), so several instances can run
simultaneously.
* examples/book/socket/plug.cc:
* examples/book/socket/socket.cc: Give plug and socket different application
ids, so they can run simultaneously. Bug #681323.