GDBus: ignore qualified XML tags and attributes

Ignore qualified (in the XML namespace sense) tags and attributes when
parsing D-Bus introspection XML.

This will allow people to add custom tags and attributes to their D-Bus
interfaces without tripping up GDBus.

https://bugzilla.gnome.org/show_bug.cgi?id=665634

gio/gdbusintrospection.c

@@ -1789,7 +1789,7 @@ g_dbus_node_info_new_for_xml (const gchar  *xml_data,
 
   data = parse_data_new ();
   context = g_markup_parse_context_new (parser,
-                                        0,
+                                        G_MARKUP_IGNORE_QUALIFIED,
                                         data,
                                         (GDestroyNotify) parse_data_free);