show vertices only for selected way

I think we should strongly consider this again. It has several advantages:

* Reduced visual clutter, less overwhelming for new mappers.
* Improved performance. From what I've seen lately, vertex rendering is the rendering hotspot.
* Significantly reduces the chances of accidentally dragging a vertex when you were trying to pan.
* Fixes #569.
* Provides an elegant solution to #750 -- we can copy what [P2 does](https://github.com/systemed/iD/issues/750#issuecomment-14196201).
* Makes it easier to implement simplification for ways when rendering, a potential performance improvement -- we can simplify non-selected ways without having to worry about them not matching up with their vertices.