Additions to GInterfaces

[Mike Kestner <mkestner novell com>]

When auditing the new API in 2.8 for Gtk#, I noticed that a property and
signal were added to the FileChooser interface.  Adding anything to an
interface is a non-compatible change, because any class implementing the
interface must be updated to add the new API members.  Whether a C
compiler would complain or not, conceptually it's a breaking change and
will most likely cause trouble for any language binding that supports
interfaces.

Since we haven't added GInterface registration to Gtk# yet, I'm going to
just let this break occur in Gtk# as well.  It's only non-compatible for
implementors, not consumers.  We do plan to support GInterface
registration in an upcoming release though, so I would like to request
that interface stability be guaranteed in future releases.

-- 
Mike Kestner <mkestner novell com>