Save last cursor to config file

[derhofbauer]

Since, in theory, filesListFolderContinue gives as all changes since since the last cursor, we should store this cursor persistently.

This can be used to start downloading files while local files are indexed and a new file list is fetched from the server. In this scenario files can only be downloaded when the local file has already been indexed or does not exist.

The procedure works even when we start syncing both ways.

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For one-way sync we just need to store the last cursor and on restart fetch all changes since this cursor and produce them in our file system. We do not need a complete file list. It might be safer though, to fetch one on startup and then produce the changes based on this file list.

[derhofbauer]

This is in fact fixed already, but kept open for note purposes.