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.
The previous API revolved around methods on TextBuffer for querying
and manipulating markers based on their id. Now marker creation
methods return marker objects. These are still retrievable by id so
they can be dealt with across serialization boundaries in the future,
but you deal with them directly as objects.
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.
This is needed by the snippets package, which needs to know where the
cursor was previously when it moves to decide whether to cancel the
current snippet.
This is used by cursor to only autoscroll when the marker head is
explicitly moved, so that passive movements caused by buffer insertions
don't trigger autoscrolling.
A "marker" is basically like a persistent selection/cursor composite,
having a head and a tail. The "head" is like the cursor in a selection,
and the "tail" is like the part of the selection that doesn't move. My
goal is for markers to be the only construct used to track regions
in the buffer. I want to replace anchors with them.
