Move selection modification out of view.rs

[cmyr]

A user of the xi-core library should be able to use and modify selections without needing to use a View.

The main challenge here is that we need to introduce some abstraction for representing the things that are currently provided by the view, such as navigation up and down by arrow keys, which depends on line breaks.

My inclination is to introduce a trait that describes this behaviour.

For more details see #1187; also a version of this exists in my xi-modal branch (used for the rust playground): see here.