Problem: Text properties don't always move after changes.
Solution: Update properties before reporting changes to listeners. Move text
property when splitting a line.
Problem: Cannot flush change listeners without also redrawing. The line
numbers in the list of changes may become invalid.
Solution: Add listener_flush(). Invoke listeners before adding a change
that makes line numbers invalid.
Problem: No docs or tests for listener functions.
Solution: Add help and tests for listener_add() and listener_remove().
Invoke the callbacks before redrawing.
