This is required to add objects in the "Types and Values" section and
in the API index. Later, we may want to add useful content in those
empty comments.
Add a generic NMSettingTunnel which describes properties of tunnels
over IPv4 and IPv6 (GRE, SIT, IPIP, IPIP6, IP6GRE, etc.). Since those
tunnel types all have similar properties it makes sense to have a
single setting type for them.