Deprecate QByteArray <=> NSData converters for 5.3

The NSData conversion functions have been moved to QtCore so deprecate
their use in QtMacExtras.

Change-Id: I0e78b32c803a2856366d15302c96014af3ef4c34
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>