Add progress reporting

No longer output messages directly during transcoding or tagging a file,
but use callback function of Parallel::Formanager to display all
messages when child exits.

Make first a separate processing loop for files that don't require
transcoding, while at the same time collecting the names of he files
that will be trancoded. This first loop doesn't employ parallel
processing, but the second, transconding, loop does.

Progress reporting during the transcoding loop is (re)introduced, this
time with full support for parallel processing. After 20 s of processing
the estimated finish time is also given, based on transcoding rate and
total file size left to transcode.
Update changelog to describe rev 5