CrashHandler: Opt-in instead of Opt-out

Enable the crash handler by defining the environment variable
QTC_USE_CRASH_HANDLER=1 (evaluated on start up).

Change-Id: Ib7f9fa016ab24a15082fbd90fd334bd4f2b17234
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>