Implement using Line2D for scene storage

The new scene object has a different schema, so this will not work with existing tracks unless I write a migration script, which honestly isn't that hard but what is there to save in the first place?

Also, note the convention of point line definitions. The position of points are `pos` which have properties `x` and `y`, and the endpoint IDs of lines are `pq` where the first point ID is `p` and the second is `q`. It took me a while to settle on that convention, because I really didn't like seeing numbers in properties as previously with `p1` and `p2`, and `pq` looks symmetric.