Add possibility to attach classes to line wrapper.

Closes #896

So that I don't have to say 'no' so often

Issue #898

Closes #898

As shown at GOTO Aarhus this month

Issue #881

This way, it is also used when measuring the line.

The previous approach was completely misguided.

Issue #894

Issue #892

Because laying out the whole thing again is more work than what is
saved by manipulating it in hidden state.

Issue #892

To save round-trips on layout.

Speeds up rendering by 33% on Chrome.

Issue #892

(git is the authoritative source of who wrote what)

Removes the kludge with the vertical scrollbar trying to handle
all wheel events.

Issue #810

Issue #811

It messes up parent relationships in inner nodes, and we'll re-use
the marker and widget nodes later.

Closes #890

The reason for putting them in doc turned out to be misguided

Simply use top-level functions + a plain object.

Detect when new position falls outside of a line, and retry. Also
replaces the kludgy widget compensation code.

Closes #886

Multi-line Lua comments start with "--[[", not "--["

Issue #881

By growing the vertical scrollbar to cover the whole editor when a wheel scroll
is taking place.

(This to avoid manually handling wheel events, for which there doesn't appear
to be any way that feels native across browsers.)

Closes #810

And add marker changes to the undo history, so that
un/redoing doesn't cause 'ghost' markers to appear.

Closes #882

So that, even though they share a z-index, they overlay the text and
marked spans.

Closes #883

This basically matches the CSS3 spec (except it doesn't match non-ascii characters).