add load tree option into Multifile

[borondics]

Loading many files with multifile is very useful but creating lables is cumbersome. @markotoplak made a very good point that organizing files in subdirectories can be a help filtering for the subdirectories and using them as labels.

However, loading files from subdirectories is also a bit cumbersome. :)

Suggestion: enable selecting a directory and loading all subdirectories (maybe even creating labels automatically?)...