Add one way sync

### Expected behaviour

The desktop client to have the following options:

- One way sync of **a nextcloud folder to a local folder**:
- Nextcloud files are synced/downloaded to the local folder.
- Local files are **not** synced/uploaded to nextcloud.

- One way sync of **a local folder to a nextcloud folder**:
- Local files are synced/uploaded to nextcloud.
- Nextcloud files are **not** synced/downloaded to the local folder.

### Actual behaviour
only two way sync.

### Others
I could not find another feature request that would cover the expected behaviour. #267 mentions one way sync, but _without the need of a local copy_, which is not desirable for this feature request.

In the [nextcloud help](https://help.nextcloud.com/t/one-way-sync-feature/29126) one user claims that _Nextcloud normally does not keep backups of the files by design_. Would be great if the developers comment on this claim/issue.