Make relative line counts take folds into account.

Though regular line numbering ignores folds, meaning that
it continues to show the appropriate line numbers, relative
line counts count folds as a single line for counting
purposes. We now have methods to calculate the logical
line corresponding to a given actual line, and use these to
calculate relative offsets instead of literal line numbers.
Post notifications when creating, opening, and closing folds.
Update line numbers view on fold changes.

Before, in absolute line number mode, opening/closing a
fold would take a second to reflect in the line number view,
while creating a fold would not update until the user scrolled
to trigger a re-display of the line number view. Now, all of
these immediately update the line number view.