Feature: Layers API

[bryphe]

This implements the addLayer API on the Buffer object. This is the realization of part of the strategy called out in #362 - having custom UI implement on top of a buffer.

This opens the door for several features:

• Embedded Browser - implemented as a 'layer' on top of special buffers.
• Modernize Quickfix List #730 - modernize quickfix - we can improve the rendering via a layer
• Start Screen - we can leverage this functionality to create a start/welcome screen that isn't bound by terminal limitations.
• Lowering the Bar: Gamify & Improve Vimtutor #430 - we can create a layer that does custom rendering that would lend itself well towards gamification of vimtutor.

It can also be used to handle some of our existing functionality. I'd like to refactor out some pieces of our NeovimEditor and move them into layers. Some layers that would basically be plug-and-play are the ones that are already in the <NeovimActiveWindow> in our <NeovimSurface> component:

• Definition UI / Hover UI (language service pieces)
• Scrollbar
• Errors

In addition, I could image even moving some pieces like the cursor or canvas rendering out to layers, too (longer-term). This prevents us from doing duplicate rendering work, as today, the canvas is still being rendered underneath the layer.