Use decorations API

I'm converting the gutter and highlights view to use the new decorations API.

This is good because it lets us delete a lot of functionality that's now provided by core Atom.

  - [x] Convert GutterView
  - [x] Convert HighlightsView
  - [x] Use a consolidated collection of markers
  - [ ] Update specs
Note that the rest of the changes in the method are refactoring. This line, however, is new functionality. It's here to make sure lint errors like "missing semi-colon" appear. The decorations API won't show it otherwise.