This commit moves all the remaining concerns not related to decorations
out of `DisplayBuffer` and into `TextEditor`. This means the
`DisplayBuffer` is now free to be renamed to `DecorationManager`.
I’m creating the DisplayLayer in the DisplayBuffer. In places where our
API deviates from DisplayBuffer, I’ll use the DisplayLayer directly from
as a property of TextEditor. Otherwise I’ll continue to delegate from
the DisplayLayer into the DisplayLayer to minimize impact until the
DisplayBuffer can be removed entirely.
