Added Opus as feature (__OPUS__). If user don't have new enough Taglib use libopusfile tag parser (__OPUSFILETAGS__). Also moved Opus as feature (enable with opus=1)
Inform user that we not using Taglib for parsing tags from Opus file
Add needed #ifdef to soundsourceproxy.cpp to support Opus feature if it's enabled
Added Opus to SConstruct-script
Minimum needed libopusfile is version 0.2 and there is code for parsing tags without Taglib 1.9.1 (that supports Opus). Bitrate is also corretly calculated.