Timestamps in the main window should be human-readable

As reported in the monthly "digest"... Timestamps in the main window should be more readable, as the current GMT+DST information is too much for the average user.

However is not as simple as just parsing the string when accesing the database and storing it in the treestore, as that way the ordering of the files can be incorrect. Therefore the representation of the string is what should change, not the stored data. Perhaps the textcellrenderer should be extended or overriden

Also, a configuration option could be added to switch between "local" dates and complete information, to fit each user.