core, frontends: XMLUI refactoring:

- XMLUI now use objects with 2 main classes: widgets (button, label, etc), and container which contain widgets according to a layout
    - widgets and containers classes are found through introspection, thereby it's really easy to add a new one
    - there is still the AddWidgetName helper, for example AddText('jid', 'test@example.net') will add a StringWidget with name "jid" and default value "test@example.net"
    - container can be inside other containers. changeContainer change the first parent container

src/plugins/plugin_xep_0055.py

@@ -75,7 +75,7 @@ class XEP_0055(object):
         """
         form_ui = xml_tools.XMLUI("form", title=_("Search directory"), submit_id=self.__menu_cb_id)
         form_ui.addText(_("Please enter the search jid"), 'instructions')
-        form_ui.changeLayout("pairs")
+        form_ui.changeContainer("pairs")
         form_ui.addLabel("jid")
         form_ui.addString("jid", value="users.jabberfr.org") # TODO: replace users.jabberfr.org by any XEP-0055 compatible service discovered on current server
         return {'xmlui': form_ui.toXml()}