digType designating below certain Z Levels

[AstraAmbrosius]

Instead of manually having to remove designations from digType it would be convenient if at all possible to be able to designate all of one type of tile below a certain Z level.

[myk002]

Should be very doable. The z coord here just needs to be configurable.

It might be nice if all tools that take a map bounds could use a common interface. Blueprint, for example, has a more flexible boundary selection that we could build on.