[systemsettings] Update VpnModel to use libconnman-qt VPN API. Contributes to JB#45378
Functionality from VpnModel has now been moved into libconnman-qt VpnCoreModel. This updates VpnModel to use this libconnman-qt VPN API, by extending VpnCoreModel, rather than interacting with connman directly through its dbus interface.
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