Still a couple of specs failing on this for changes that straddle the
start / end of a fold. We need a new marker invalidation strategy for
these cases.
Instead of marker-added and marker-removed events which are emitted
when markers are created/invalidated/revalidated/destroyed, we now
just have marker-created events that are triggered *only* when markers
are created for the first time. The marker itself emits a 'destroyed'
event when it is destroyed. The marker already notifies observers when
its validation status changes, so that's covered.
This prevents the editor from synchronously redrawing in specs with
cursors in invalid locations. Markers are always updated under the
hood before a change event is emitted from the Buffer or DisplayBuffer.
We still wait to trigger marker observers, but if their position gets
read, it is up to date.
There are 3 failures, which I think are attributable to the fact that
I'm not emitting events from the when the selection tail changes, and
I'm not emitting redundant events when the head is changed to the
same position it was previously, whereas before I wasn't guarding
against redundant events.
