Allow showing Nepomuk metadata inside views

Metadata like image-size, rating, comments, tags, ... can be shown
now in the view (e.g. as column in the Details mode).

Still open: The rating-information needs to be shown as stars.

In the context of this feature also the following bugs have been
fixed:
- Fix visual glitches in the header of the Details mode
- Improve the minimum column width calculation to respect also
  the headling and not only the content

BUG: 296782
FIXED-IN: 4.9.0