Rename the QtMacExtras namespace to QtMac

Change-Id: If6a01685a9fc5ff59dd02dbeeef7e8682cefae39
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>