Improve experience for Multifile/Folder Upload

[hoxbro]

Currently, the FileUpload widget looks like something from the '90s, which is "good enough" for simple cases.

A case where one could argue that it could be improved are:

1. Better feedback for the process of uploading multiple files. This could be a two-step procedure
   a) Uploading of files to the browser. If we can monitor the process of individual files, I suggest adding a progress bar per file. If not, then we can do one for all files.
   b) When the files are synchronized to the server. This could be done with a green checkmark for each file or one for all.
2. The second thing is that it is currently not possible to upload a folder/directory directly. This can solved by enabling the HTML attribute webkitdirectory. I have a POC here. A limitation of this attribute is you will get a warning message (for security re
   
   asons).

The good thing is the files will have information on the folder structure, as shown in this playground example.  

Some inspiration for how a more modern could look:

• If we want to implement it ourselves [ref]
• A finished product [filepond]