Blow away old line change handling code.

Now that folds are attributes, we’ll be dealing with
maintaining fold info through text changes separately.
Get rid of _manualFolds array.

We now track folds as attributes in our text storage.
Properly apply folds when inserting/replacing text.

This makes folds reflect properly when the user is typing
or otherwise modifying text around folds.

The only corner case I’ve spotted that’s a little broken right
now is being in insert mode and inserting a newline at the
beginning of a fold, which does funky things.