Read JSON from ffprobe

Use JSON output from ffprobe instead of string parsing to get some kind
of type safety.

For doing that, some changes were needed in FileView: Use delegates for
displaying Duration and Bitrate instead of mangling output in
Qt::Displayrole.

To reuse code, move all delegates from the new Archive to a separate
file.

And, of course, the initial objective: Show the accumulated size and
duration of selected files in the status bar from the experimental
archive.

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