audio/audiowmark: Add new port

A sound file is read by the software, and a 128-bit message is stored in
a watermark in the output sound file. For human listeners, the files
typically sound the same.

However, the 128-bit message can be retrieved from the output sound
file. Our tests show, that even if the file is converted to mp3 or ogg
(with bitrate 128 kbit/s or higher), the watermark usually can be
retrieved without problems. The process of retrieving the message does
not need the original audio file (blind decoding).

Internally, audiowmark is using the patchwork algorithm to hide the data
in the spectrum of the audio file. The signal is split into 1024 sample
frames. For each frame, some pseoudo-randomly selected amplitudes of the
frequency bands of a 1024-value FFTs are increased or decreased
slightly, which can be detected later.