R1:043e0db8d36b

When selecting implicit column positions, actually create them

Summary:
Ref T4807. This is an alternative to D10179. The problem these diffs solve is that I want to be able to reorder a column's positions without having to load the actual objects, but that's difficutl because two positions may have the same sequence number (and I think it's good that we allow that, since it makes a bunch of other stuff way easier).

Instead of using the object ID (e.g., the task ID) to reorder positions with the same…