core (common): JidSelector + behaviors various improvments: - renamed *Behaviour => *Behavior to be consistent with Kivy + moved to new "core.behaviors" modules - use a dedicated property in ContactItem for notification counter (which is now named "badge") - in JidSelector, well-known strings now create use a dedicated layout, add separator (except if new `add_separators` property is set to False), and are added to attribute of the same name - a new `item_class` property is now used to indicate the class to instanciate for items (by default it's a ContactItem) - FilterBahavior.do_filter now expect the parent layout instead of directly the children, this is to allow a FilterBahavior to manage several children layout at once (used with JidSelector) - core.utils has been removed, as the behavior there has been moved to core.behaviors