Deatach the labelResolved model in TooltipBehavior.onDeatch()

Commit 5c4120dd1d17f7a819a544069a098bd1b93d7fc2 added a labelResolved model field, but does not detach it in onDetach().

This causes Wicket's NotDetachedModelChecker to fail any serialization of a page that contains TooltipBehaviour.