Correct base type check in Container_Class::remove_callback()

* gtk/src/container.ccg (Gtk::Container_Class::remove_callback):
In order to test for the abstract GtkContainer parent class, actually
compare against the parent type instead of making invalid assumptions
about the derived type.  By means of the alternative Glib::ObjectBase
constructor, it is possible to create a custom GObject type different
from the standard one.