Nepomuk magic for downloading files

With this patch, the browser
- adds downloaded files to Nepomuk,
- tags them as "Download" (not translated -- maybe we should do that?),
- and adds them to the current Activity, as Ivan described and
- adds the source URL of the file as comment to the downloaded file's
  metadata

Ivan, could you check that the activity part works?

CCMAIL:active@kde.org