Change camel-case header <QMacFunctions> to <QtMac>

Name the header after its namespace, to reduce the number of names
developers must remember.

Change-Id: I433263d8a0d70993d5929adeb1198f3e05df8b36
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>