Video library: Import speedups

These are really bug fixes.  We do three silly things:

1. When searching for trailers, we also open up any .rar/zip files in the folder - unnecessarily so as we don't match .rar/zip files anyway.
2. When adding movies we invalidate the path hash (and parent hashes etc.) as the movie is deleted even though we're just updating info - the hash needn't change here.
3. When setting the hash we do a (potentially expensive) directory existence check.  This is compounded by the invalidation in step 2.