Debugger: Limit QObject name dumper to a more-than reasonable maximum

GDB can crash on to big chunks. Since this here is a convenience
feature only, hard-limiting it to length 200 seems ok as work-around.

Task-number: QTCREATORBUG-12330
Change-Id: Ia6ebf16f3528d91020d19851641f42c1e421fe15
Reviewed-by: Eike Ziller <eike.ziller@digia.com>