Cache layout information for selection navigation

[davidbalbert]

Some Selection movements – moving up and down by line fragment or by page – are layout dependent. Right now, we only cache layout information for the portion of the document that's visible in the scroll view, so if the caret is outside of the scroll view when you navigate, layout will have to happen then.

This is not an issue with a single caret, but once we support multiple carets, this could get costly fast (imagine having 10,000 carets and pressing the down arrow).

Even if we cache those, an operation like "add a caret at the beginning of every line" could be very expensive if we layout all the lines right then, so we might want to think about doing some of the layout when you create a new caret in the background.