networking: add ability to manually add wifi networks

[soramanew]

Currently, you can only connect to a wireless network that is exposed in the NetworkDevice::networks property. This PR adds the ability to connect to networks that do not expose their SSID, or that don't show up in the networks list for whatever reason.

This is done via a new WifiDevice::addNetwork method, which takes the SSID, security type and options, and whether or not the network is hidden.

Example usage:

Item {
    required property WifiDevice wifi
    
    Component.onCompleted: {
        wifi.addNetwork("some_ssid", WifiSecurityType.Wpa2Psk, {
            psk: "some_password"
        });
    }
}

[cpwrs]

This looks good and I have a few ideas/questions.

Is there a case for adding a network that isn't hidden?
We currently show a network as long as there exists an AP OR available connection for a given SSID.

I think we should add a hidden flag on WifiNetwork.

I'm working on ways to clean up the NMSettings mess so that we have a general settings class and don't have various maps to parse and validate. Hopefully that can end up replacing the credentials argument.

[cpwrs]

We currently show a network as long as there exists an AP OR available connection for a given SSID

Some work needs to be done to expose org.freedesktop.NetworkManager.Settings to list all networks including unavailable and hidden.

Then adding an available boolean on Network and/or a list on the NetworkDevice will make it clear which networks have found APs.