Offer anonymous connection to read-only WebDav server

## What Happened

Set up a WebDav server with unauthenticated read-only access. Attempted to connect to this with the ConnectServer dialog. Could not connect until a user name and password was entered - the user had to be registered locally.
The server could be connected to by entering the address `dav:///webdav` into Files pathbar.

## Expected Behavior

An option to attempt anonymous connection should be offered as it is with FTP.
Rather than adding a further option to the combobox, radio buttons for "anonymous" and "authenticated" could be offered.

## Platform Information

Elementary Odin daily