Replace NMNetworkMenuItem with NMConnectionItem, based on
NMVPNConnectionItem, and replace NMDevice with NMConnectionSection
and NMConnectionDevice.
Since this rips apart NMDevice, and since wi-fi should not be
connection-based, we'll temporarily remove NMDeviceWireless. We'll
add it back in a later commit, along with the new Wi-Fi dialog.
https://bugzilla.gnome.org/show_bug.cgi?id=704670