Wifi user agent does not support multiple networks

Right now the API allows only one global user agent to be accessed from QNetworkSettingsManager. The problem here is that if I have several networks available with different PSK the called slot does not know which of those to provide because the network to be connected to is unknown.

Solution could be to provide network name and authentication type when calling the slot (like connmanctl does). Or provide a slot per QNetworkService, this way I can connect a passphrase function per service.

Also I would suggest a name change. Because 'showUserCredentialsInput' is too GUI specific. In my case I provide the PSK from a config file without using an UI.