Issue title: Checking that a TURN server is working.
 Issue body: I am confused, and don't seem to be able to find this in the documentation.

I am using Linphone behind several NATs, and often the connection is bad.

In order to improve the connection, I installed a `coturn` TURN server on my VPS, and want to make Linphone use it for relaying calls. As I am the only user to use this TURN, I set up my login and password on the command line, and set the realm to the VPS domain name.

In the "proxy accounts" settings, I

- flipped "Enable ICE" 
- flipped "Enable TURN"
- in the STUN/TURN server, wrote the domain name of my VPS machine
- in the login/password, wrote the `coturn` login and password.

But I am not getting any confirmation in the log files whether TURN is successfully used. 
Moreover, when I reopen my proxy account configuration window, I see that the password is not saved.

I have sent the logs to apineda@example.net

 Comments: 
 Comment 0: This is strange that you don't retreive the TURN configuration as it is stored in .config/linphone/linphonerc (in a auth_info section)
 Comment 1: Can I help somehow? Some more debugging information?

{'source': 'GithubIssues', 'repo_name_with_owner': 'BelledonneCommunications/linphone-desktop', 'issue_id': 640136410, 'repo_stars': 312.0}