Interface macro usage

[Mike Kestner <mkestner speakeasy net>]

There are several interfaces in gtk+ which don't use the
G_TYPE_INSTANCE_GET_INTERFACE macro in favor of a more longhand
approach. :-)  Specifically, GtkCellEditable, GtkTreeDragSource,
GtkTreeDragDest, GtkTreeSortable, and GtkTreeModel. Would it be possible
to update these to use the macro to facilitate interface parsing for
language bindings?  I can provide a patch, or just commit a change if
it's acceptable.

Regards,
Mike