Adds FEC support for the SILK codec. Minor clean-ups in EncodingConfiguration and MediaConfigurationImpl.

Changes the EncodingConfiguration instance stored in MediaServiceImpl to one which contains global settings. Changes the names of some methods in EncodingConfiguration and MediaService to make their meaning more clear. Adds storeProperties() methods to EncodingConfiguration, makes EncodingsPanel use them.

Adds support for the IETF Opus codec (patch by Boris Grozev). Fixes a problem that prevented Jitsi from using formats that are disabled by default (pach by Boris Grozev)

Applies a patch from Boris Grozev that allows setting different codec configurations to different accounts. It is now possible to set entirely different codec preference for every account.