Sort modified date chronologically

In my locale datetime.strftime('%c') is a string beginning with the dayname of the week and therefore when we sort on it, the items are ordered by the day of the week on which they were modified, and not the Unix timestamp.

Since we're already overriding sort behavior, I considered keeping the original timestamp as hidden information, but it seems more in keeping with Qt's design to use a delegate to alter how it's displayed instead.