Add pooled as a property to ItemDelegate

ItemDelegate is typically used in a view, so to make things a little bit simpler without having to manually create the right kind of attached property for each delegate in different views (ListView/GridView/TableView), maybe it could have pooled property that is updated based on onPooled/onReused notifications from the attached property.

Or perhaps to save some memory, maybe the view could do that on its own if a delegate has "required property bool pooled" so no attached property need to be constructed for every delegate.