For better typesafety on the user side (and optional) for items with uniformly
typed children.  Use it for UniformTreeModels, and consequently WatchModel
to get rid of some of the static_casts there.

Change-Id: Ic20e507036e180c24997b236230f0f71b285202c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>