Also extract creation date/time

Video creation time is now extracted using ffprobe as suggested in bug 472520. The extractor and job were also renamed.

To test this without the patch, add one or more new videos to the demo, rescan, and refresh. The added videos are assigned the file modification time.

Repeat with a fresh demo running the patch; the files are assigned the creation time reported by ffprobe.

In both tests, the date and time for the demo's movie.avi is unchanged since it was user-configured.

CCBUG: 472520

CCBUG: 511440