Utilize the new Q_OS_MACX define.

All occurrences of `#if defined(Q_OS_MAC) && !defined(Q_OS_IOS)` have
been replaced with `#if defined(Q_OS_MACX)`.

Change-Id: I5055d9bd1845136beb8ed1c79a8f0f2c0897751a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>