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>

gio/gdbus-codegen/utils.py

@@ -48,6 +48,13 @@ def lookup_docs(annotations):
     else:
         return s
 
+def lookup_since(annotations):
+    s = lookup_annotation(annotations, 'org.gtk.GDBus.Since')
+    if s == None:
+        return ''
+    else:
+        return s
+
 def lookup_brief_docs(annotations):
     s = lookup_annotation(annotations, 'org.gtk.GDBus.DocString.Short')
     if s == None: