Mike Fleetwood authored Jan 11, 2016 and Curtis Gedak committed Feb 12, 2016

Write a generic progress bar class.  Has the following features:
* Has separate progress and target numbers, rather than a single
  completion fraction, to enable the the next feature.
* Optionally generates text reporting the amount of data copied using
  the progress and target numbers like this:
      "1.00 MiB of 16.00 MiB copied"
* After running for 5 seconds, also add estimated remaining time.
  (Waits to allow the data copying rate to settle down a little before
  estimating the remaining time).  Looks like this:
      "1.00 MiB of 16.00 MiB copied (00:01:59) remaining)"

The ProgressBar class is not driving the visual progress bar yet.  It
has just been added into the internal block copy algorithm and generates
debug messages showing the progress bar is operating correctly.
Debugging looks like this:

    DEBUG: ProgressBar::start(target=2.0636e+09, text_mode=PROGRESSBAR_TEXT_COPY_BYTES)
    DEBUG: ProgressBar::update(progress=1.30023e+08) m_fraction=0....