- Fix several issues in the document checker:

- Fix several issues in the document checker:
  * when relocating a clip, update its resource in slowmotion
    clips too;
  * avoid deleting consecutive non-found clips if the user
    didn't select them;
  * when deleting clips references, do not delete only one
    of them;
  * avoid skipping one reference for any deletion;
  * when removing a clip, take care of its slowmotion and
    audio/video only producers too;
  * don't leave orphaned filters when removing playlists
    entries;
  * fix a possible crash.

svn path=/trunk/kdenlive/; revision=4878