tests: Move TestSignals from test_everything into test_signal

Move these tests into a more meaningful location.

https://bugzilla.gnome.org/show_bug.cgi?id=701843
refactor: Move builder connection utilities outside of Builder class

Move _extract_handler_and_args and _builder_connect_callback into module
scope for re-use by GTK+ Composite Templates.

https://bugzilla.gnome.org/show_bug.cgi?id=701843
demos: Cleanup CSS accordion demo to use a loop for adding buttons

Also use Gtk.main_quit directly since it has been overridden to accept
extra args.