Move property and signal creation into _class_init()

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 is a backport of commit efcb0f9f for 2.28.x. This allows old pygtk
applications to work with pygobject 2.28.x and glib 2.35.x.

https://bugzilla.gnome.org/show_bug.cgi?id=694108