Split a base NMSettingIPConfig class out of NMSettingIP4Config and
NMSettingIP6Config, and update things accordingly.
Further simplifications of now-redundant IPv4-vs-IPv6 code are
possible, and should happen in the future.
Add key-value attributes to NMIPAddress and NMIPRoute, and use them to
store IPv4 address labels. Demote NMSettingIP4Config:address-labels to
a D-Bus-only property, and arrange for :addresses setter to read the
labels out of that property when creating the addresses.
Merge NMIP4Address and NMIP6Address into NMIPAddress, and NMIP4Route
and NMIP6Route into NMIPRoute. The new types represent IP addresses as
strings, rather than in binary, and so are address-family agnostic.
