editor/view: handle changes without a complete refresh

Upon receiving a change, the View currently ignores the contents of the Change struct and does a complete refresh of the text and marks. However, add changes with Text set and all delete changes can be handled locally without a round-trip to the editor.

Start in the editor/view change-handling path and inspect how the Change struct is currently processed. Trace the complete refresh and editor round-trip, then verify that add changes with Text set and all delete changes are handled locally without that round-trip.

Written by the indexing model from the issue text.