Improve performance of Dolphin:

Improve performance of Dolphin:
- When changing a directory the receiving of the meta data is done now with a short delay. This gives the directory lister the chance to show the directory as fast as possible.
- Prevent that DolphinInformationPanel::reset() is invoked when changing a directory. The method only needs to get invoked if an item has been deleted or renamed.
- Cache an unknown file item. This improves the startup time of Dolphin (-> KFileItem constructor is invoked only once instead of up to 6 times).

svn path=/trunk/KDE/kdebase/apps/; revision=991840