Update to diagrams-lib-0.7

`diagrams-lib` has between `0.6` and `0.7` changed in two points relevant to this package:

- To get a "concrete location" trail / segment / etc., you previously had to pair it as `(Point, Thing)`. There is now [the `Located` type constructor](http://hackage.haskell.org/packages/archive/diagrams-lib/latest/doc/html/Diagrams-Located.html#t:Located) which handles this in a better way.
- Trails are now not a simple union ADT with `Loop` and `Line` constructors anymore, but [a GADT wrapper around a type-indexed `Trail' Closed` or `Trail' Open` type](http://hackage.haskell.org/packages/archive/diagrams-lib/latest/doc/html/Diagrams-Trail.html#v:Trail).

Both changes brake this backend package. I adopted it so it works with the new API. It is, however, now not compatible to `diagrams-lib-0.6` anymore.