Implement pull download indicator

When pulling an image from the registry is requested or required, use
the streamed progress information during the pull to show a download
progress indicator, based on the average of the download progress of all
image layers (as such, it's possible the value may decrease at the
beginning until all layers become "known" through the stream).

Signed-off-by: Maxime Petazzoni <max@signalfuse.com>