Layers

Some differences between https://github.com/Financial-Times/ft-origami/issues/183 and these spec suggestions

 - haven't written up the idea of having modules able to provide js mixins via files other than main.js. Will put that in another PR as it's not essential to this approach, and also relates to e.g. modules providing a choice of middlewares

- dropped the notion of using capture for events as I felt it was a bit confused and required too many tweaks to other parts of the events spec. Have instead gone for having a class `o-layers__context` for defining a context for events. It feels more flexible and less magicky

@kavanagh @triblondon @dan-searle @matthew-andrews