Improve search speed a little bit

Improve search speed by caching loaded .desktop files. Before this commit each time the result list is retrieved, each item' .desktop file was loaded, parsed and additional information retrieved. Then this result list was sorted what caused the .desktop files being loaded, parsed etc. or each item. To avoid this each .desktop loaded will be cached so it will be only loaded once. At my slowest machine this reduced the mean search time for the term 'a' from 350ms to 240ms.