[Enhancement] Make sending and drag'n'drop of folders possible

[schlagmichdoch]

What problem is solved by the new feature
Selecting a folder via the native file selector is not possible. Also, when drag n dropping a folder containing files nothing happens.

Expectedly, either the contained files would be sent individually or the complete folder is sent as a zip archive.

Describe the feature
When sending a folder as a zip archive, a service-worker should be used for zipping in order to prevent exceeding the available RAM.
When sending the contained files individually, all files should be extracted as in this stack overflow answer:
https://stackoverflow.com/questions/51850469/file-and-directory-entries-api-broken-in-chrome/51850683#51850683

Optionally:
Before sending a folder, a dialog opens to select whether the folder should be sent as a zip archive or whether only the contained files should be sent.