Sync Heading folds in Edit/Preview and Outline when panes are linked

Use case or problem

Heading folds in edit mode don’t match heading folds in outline. i.e. When I fold a heading in one, it remains unfolded in the other.

Proposed solution

We already have pane linking :smiley:. It would be really nice if pane linking also synced heading folds across Outlines/Edit Mode/Previews.

Added benefit: Users can toggle this syncing, simply by linking or unlinking just as we did before.

(In general, pane linking should match the current state across any panes that are linked.)