[DVD/OMXPlayer] Fix auto-selection of audio and subtitle streams.

XBMC's audio and subtitle language is stored as an ISO 639-2 T code, but a video's meta data could also store it as an ISO 639-2 B code which could differ from the T variant.
This problem is solved by using a new generic language compare method.

This PR replaces the abandoned PR #2722.
@popcornmix can you have a look at this please, since OMXPlayer is involved?