Spotlight: file modification date, bug #545

The Spotlight module misleadingly updated a files modification date
when the client requested an "kMDItemLastUsedDate" attribute update.

It's questionable whether we're expected at all to map and modify
attributes to filesystem metadata, but my guess is we are.

As a fix, use "kMDItemFSContentChangeDate" attribute instead of
"kMDItemLastUsedDate".

Fixes bug #545.