Improve the performance when creating previews for thousands of items by the followin...

Improve the performance when creating previews for thousands of items by the following strategy:
 - The previews for currently visible items are created before
   the previews for invisible items.
 - If the user changes the visible area by using the scrollbars,
   all pending previews get paused. As soon as the user stays
   on the same position for a short delay, the previews are
   resumed. Also in this case the previews for the visible items
   are generated first.

svn path=/trunk/KDE/kdebase/apps/; revision=815480