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.