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?)...

[raul-freitas]

@borondics and @markotoplak , I agree with Feri that sometimes is quite cumbersome this task. Last year we were discussing some possibilities and I suggest something like the picture below:

What do you think?