Xi-editor: Split into xi-core and OS X frontend

[theduke]

Since there are multiple frontends popping up, and the goal is a independent structure,
I think it would make sense to split into a xi-core repo with the server and a separate xi-cocoa project for the frontend.

[Eh2406]

Could there be a list of frontends somewhere?

[raphlinus]

I have the repository created, just need to do the split.

[theduke]

How's that coming along, @raphlinus ?

While there are relatively few issues at the moment, it's confusing to separate what's related to the core vs the Cocoa frontend.