nsISelectionListener isn't sufficient because it only notifies that the selection has changed.
A11y needs to know specifically which ranges were added or removed.
Previously, we handled this specifically for spelling errors in mozInlineSpellChecker.
That code has been removed and replaced with a more general approach.
Since we now have these notifications, rather than using nsISelectionListener just to fire events, we now use these notifications for that as well.
This avoids the need to add additional nsISelectionListeners, which would get messy particularly for custom highlights where there can be an arbitrary number of selections at any given time.
Differential Revision: https://phabricator.services.mozilla.com/D217068
bd70ec7bf6