Support tunnel=culvert

[bryceco]

Currently there is no direct support for adding tunnel=culvert to a waterway. iD contains a hack to convert tunnel=yes to tunnel=culvert when the object contains a waterway=* tag, but it would preferable for this to be built into the structure_waterway field itself.

[tyrasd]

I'm not sure if this was changed since this issue was opened, but one can set the tunnel type also for waterways in the "structure" field:

Do you think something should be changed here?