progress report after each transcode (step 4/5)

Display total flac file size to convert before transcoding
begins (in units of MB), as well as a progress report after
each transcoded file (as in "Processed 3/1267 files."). This
is accomplished by extending the callback code that runs
every time a child exits.

The timing and estimated finish time of the transcoding job is
implemented by extending this callback code further in the
next commit, step 5/5.
Merge branch 'feature/report_progress' into develop
estimate time remaining (step 5/5)

Present timing of each transcode to 0.1 s resolution and also,
after 20 s of transcoding, display the estimated time remaining
for the whole job. The latter time is extapolated from the
total file size converted so far and from the time elapsed since
transcoding begun.

The estimed time remaining is updated each time a file is
transcoded. The accuracy of the estimation will successively
improve during the job as more and more files are transcoded.

This is the fifth and final changeset for the step-by-step
implementation of progress reporting.
Merge branch 'feature/report_finish_time' into develop