Allow Cells to react to item property changes via apply

Currently a ListView cell created as a functional component can use the `apply` method/attribute with '*' or 'onItemChanged' to update itself when `item` is replaced, but it can not track changes of an already assigned item, even if that item fires change events. To enable this Cell must observe the assigned item and fire some kind of mutation event. Alternatively @prop or the tabris event system could be extended to support event "bubbling" along parent/child relationships (either widget hierarchy or property hierarchy).