Display timestamps in files tab

[fonnesbeck]

This is a feature request to display file timestamps in the "Files" view of the notebook. There is an awful lot of whitespace in the view, and a useful piece of information would be to show the file timestamps, and perhaps be able to sort by date modified.

[minrk]

The information is already present in the data we use to populate the list, so it should be a Simple Matter of UI™