Add support for org.gtk.GDBus.Since annotation

And use this for a) documentation purposes; and b) to preserve C ABI
when an interface is extended. See

 https://bugzilla.gnome.org/show_bug.cgi?id=647577#c5



for more details. Also add test cases for this.

Signed-off-by: David Zeuthen <davidz@redhat.com>