Untangling Dependency Cycles

[Iron-E]

As noted in #270, it seems like we have a dependency cycle (note the double-ended arrows):

The best way I can think to fix this is the following:

• Move bufferline.render(), bufferline.update(), bufferline.update_async() to bufferline.render.
• Move all methods in bufferline.state requiring a bufferline.update() to bufferline.render
  • Alternatively: remove calls to bufferline.update() from bufferline.state, and add methods which wrap around them and call bufferline.update() to bufferline.render
• Move bufferline.jump_mode.activate() to bufferline.render.activate_jump_mode() (bikeshedding on the name is welcome)

That would make the dependency tree look like the following:

• bufferline.jump_mode is now like bufferline.state but for :BufferPick— it holds data but doesn't render.
• bufferline.render now controls all code for rendering and updating &tabline
• bufferline.state holds information about state, but now never renders

What are your thoughts?

[romgrk]

Graph looks nice :) let's do it when we have time.