We must not add class interfaces after g_type_class_ref() has been called the
first time. Move signal and property creation from pyg_type_register() into
pyg_object_class_init(), and drop the hack of registering interfaces twice.

This changed class initialization order now exposes GLib's warning about
unknown signals, so adjust test_signal.TestGSignalsError.test_invalid_name() to
not abort on that.

https://bugzilla.gnome.org/show_bug.cgi?id=686149