Add Opus SoundSource to Mixxx

As Mangatune is currently supporting Opus releases and it's standardized by the Internet Engineering Task Force (IETF) as RFC 6716 is natural that Mixxx support for Opus. 
This is 90% made after Ogg/Vorbis SoundSource as libopusfile is nearly 100% compatible with libvorbisfile by API wise (of course there are name changes from ov_ space to op_ space).
Tested with setup:
* Libopus 1.1
* Libopusfile 0.4 (Libopufile minimum is libopus 1.0.3)

There ain't much Opus audio available in internet so I tested these with converted FLAC files or if you have Mangatune subscription you can download Opus easily from there. Currently this is always compiling but It can be tuned as feature (I'm just not that good with scons..)