Slate: change the word "state" to "value" everywhere

[ianstormtaylor]

Open to discussion on this. I initially took the word state from Draft.js, but I think it ends up conflicting with React in confusing ways—both in terms of users understanding what "state" we're referring to, and in terms of naming conflicts for variables in the codebase.

I think value might be nicer, since it mimics native controlled <input> and <textarea> and we could even make editor.value a getter for super easy access. And then setValue is available (right now we can't make our own setState function).

Would do it in a backwards compatible way with deprecations hopefully, so to have the least negative impact when upgrading.

[ianstormtaylor]

I think I might YOLO do this one very soon, since people are only becoming more dependent on the state word if they're serializing stuff to JSON and storing it.