Debugger: Add dumpers for QXmlStreamStringRef and QXmlStreamAttribute

Useful for debugging debugger tooltips.

Change-Id: Ief1f98e0baa132c23d0886aae415a5f3da0294c3
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>