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.

Remove Deinterlace routines from old place (modules/codec/ffmpeg/video_filter.c), because they have moved to their own file (modules/codec/ffmpeg/deinterlace.c).