Implement the new Opus loader.

OpusCodec uses engine/framework/Filesystem to read the audio files.

opusfile is used to decode the audio data.

All the relevant data is returned in a AudioData struct.

In case of an error an empty AudioData is returned.
Remove snd_codec*

The old sound codec loader are removed.
CodecFactory and {Wav,Ogg,Opus}Loader
are used in their place