Don't show checkbox for Creator if root on Linux and Mac.

Re-enable the check reverted in 46afe1f790, but also check for isRoot() in installationFinished().

Change-Id: Ic27e6f657d0506271d940f1c89a8bd7034bb3543
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>