Add level/layer support

[Dimitar5555]

Description

Currently when somebody is mapping malls with multiple floors or underground features (like shops in pedestrian underpasses or metro stations) users have to see all other elements that are not on that level (like roads, crossings etc.) The same is true when you edit ground features. It's easy to connect something to something else from a different level by mistake.

My proposal is to create some sort of UI which shows you on which level/layer/floor you are on and show things that are only on it. You should be able to add new features to it and iD should automatically add level/layer tag.

Screenshots

A good initial design could be the one used in iD-indoor.

[Kreijstal]

I would like this to get implemented, there was a guy that tried it but gave up #3097