Backport from trunk [24695]: set qmin=3 in ffmpeg encode when bitrate is set and user hasn't defined qmin/qmax self

Backport of from trunk [24676]: Make default tradeoff for bitrate instead of quality when CBR is requested.

Prevent change of audio channels number when mp4a is used and the mixers put 2 channels out. This is a hack and should be solved in a better way.

Add a little note about other 3rd party libs normally build in contrib being available in the medibuntu repository.