existing code was very stupid. - all ID pointers for clipboard strips are handled uniformly. - clipboard stores a duplicate ID pointer which are restored on paste. - restoring pointers... -- use ID's that are still in the database (copy&paste within the same file). -- fallback to name lookup. -- fallback to loading them from the original filepath (movie-clip and sound only). also fix bug pasting where initialing the sound wasn't done if there was no frame-offset.