Use SynchronousProcess::run and SynchronousProcess::runBlocking.

Using SynchronousProcess::run in ShellCommand::runSynchronous
is generally safe, since this method is usually called in its
own thread. This makes the processing of cancel requests work
again and makes output available via signals.

Task-number: QTCREATORBUG-16532
Change-Id: I6907194bd00cd5713673d9b1911978d69b996a80
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>