This PR moves the check from the FTP backend into shared code, so all backends will now check for read/write permissions when using the "Test" button.
For situations where write permissions are not required (like restore or verify) there is a flag on the v2 API that indicates if the connection was established, so the UI can show a message suggesting that it is possible to proceed, if the current UI operation does not require write permissions.
This fixes#2473
Kenneth Skovhede
6031b4de8b