Render PDF link decorations as dashed underlines rather than boxes

So they look more like (old-school) web links.
The underlines are always horizontal and underneath the area of
occurrence though; rotated link text has not been tested.

Change-Id: I4fc01d88367b0cc9bbc23e9f85a3b42efb271fb8
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>