Support downloading-changing-reuploading a file

[christianliebel]

A request I receive pretty often is to be able to download a file to a temporary directory, use the default editing program to perform some changes, and then re-upload the updated file. This is for example needed by applications that offer cloud-based storage.

This would require that:

• Files can be saved to a temporary location (OPFS?)
• Files can be launched (#394)
• File changes can be determined (#72)