[FEATURE] Add a directory-selection widget

[erezinman]

Add a directory-uploading/selection support to widgets.
Specifically, you can add the webkitdirectory option to the FileInput class as an option. It is supported on all the major browsers quite some time.

[philippjfr]

Would be happy accepting a PR for a DirectoryInput widget.

[jbednar]

Personally, I'd expect to see it as an option for FIleInput rather than as a new widget, but I guess either is fine.

[philippjfr]

That cannot work, they will be distinct files each of which will be uploaded as a distinct base64 string and exposed as a list of bytes objects on the Python end.