Changes inspired by whitespace package

[kevinsawicki]

No description provided.

[kevinsawicki]

Having this on Editor feels weird to me, since insertText uses selections and this does not. To me Editor methods are all about operating on selections/cursors. And this method plus deleteBufferRow deviate from that conceptual model.

[probablycorey]

We can go that route and remove all non-selection/cursor based methods from editor. But
I think it is more of an internal implementation difference than practical difference for package authors.

It comes down to: Do we want package authors to think about whether they need to call a method on an Editor or an Editor's Buffer? Or do we just want to point them towards the Editor's API.