Support Gtk.Template on sub-classed Python classes

If Gtk.Template is applied to a Python class which is a sub class of another Python class then instance initialisation fails with the error message: "gtk_widget_init_template: assertion 'template != NULL' failed".

When initialising the GObject instance which corresponds to Python object that is being created, pygobject_g_instance_init() is called once for each Python class in the inheritance hierarchy of the Python object. This method then calls __dontuse_ginstance_init__() on the Python object which in turn calls GtkWidget.init_template(). However this method must only be called once the GObject instance is fully instantiated because it is only then that the template member is initialised. This means pygobject_g_instance_init() should only call __dontuse_ginstance_init__() during its call to initialise the most-derived class.

In order to achive this, this commit adds a check which compares the current type id of the GObject being initialised with the type id of the final class of the object. Only if these to are equal __dontuse_ginstance_init__() is called.

This fixes #400.