Make the desktop client reupload a given file when requested by the remote

## Feature request

1) The nextcloud server might provide a dav property `{http://nextcloud.org/ns}request-upload`. That property might exists, or not. We need to request it during the propfind request for the sync. Either's either 0 or 1.

2) If request-upload is one, the client should check if we have a local copy of the file (either classic mode or vfs) and if so, we should upload that copy. It should use exactly the same logic as when the watcher detects a change for a file the file system. Alternative idea: If, for whatever reason, it's to complicate/impossible to queue an upload during the discover phase, we could also consider to just modify the mtime of the local file and let the watcher take over.