The gateway is a global property of the IPv4/IPv6 configuration, not
an attribute of any particular address. So represent it as such in the
API; remove the gateway from NMIPAddress, and add it to
NMSettingIPConfig.

Behind the scenes, the gateway is still serialized along with the
first address in NMSettingIPConfig:addresses, and is deserialized from
that if the settings dictionary doesn't contain a 'gateway' key.

Adjust nmcli's interactive mode to prompt for IP addresses and gateway
separately. (Patch partly from Jirka Klimeš.)