Add paramnb support for Foldername parameters

[thoth291]

In one of my current projects I'm trying to wrap a shell script generation into paramnb widget.
That requires define a folder for input data.
Currently I'm doing to like this:

using this code:

multiple_files = param.MultiFileSelector(path='*')

The problem is that I don't know how to check if selected PYTHON is a folder or a file.
Is there any way to make some validation of selection or restrict the globbing to show only directories?

Thanks in Advance!

[jbednar]

This would be a case where you'd use the Foldername parameter, which only allows folders as values. However, when you do, you'll see that ParamNB does not currently do anything special for Foldername parameters (only a textbox), so ParamNB will need to be extended to add support for exposing the correct ipywidget for this case. We'll do that eventually, but probably not soon, and meanwhile we'd be happy to accept a PR for it.