Prevent user code from hanging qbs during resolving.

Long-running commands are handled already, but badly written project
files could still hang qbs with e.g. infinite JS loops on the right
hand side of a binding. Such code can now also be interrupted.

Change-Id: Ie0d114bd37d540e764d5ec5bb323c91bfd64a67a
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>