Nsdateformatter - again

Dammit, so you merged the last pull in too quick @skurfer ;-)

I found another place where the `NSDateFormatter` was wrong - in the **Scanned** field of the 'attributes' tab of a catalog entry (the Prefs).
This is just a pull that changes that and adds one small change:

For Catalog Groups, if they don't have an actual `.qsindex` file (i.e. they're just a group and not a catalog entry) the **Scanned** date would show up empty. What I've done here is just use the most recent scanned date from any of the group's children.