* lisp/textmodes/text-mode.el (center-line-mode--track-changes):
New local variable for storing the id of the change tracker registered
for the current buffer.
(center-line-mode--track-changes-signal): New function to be called by
the track-changes library whenever there is a change in the current
buffer.
(center-line-mode--track-changes-function): New function called from the
above signal function, iterates over the lines of the modified region,
calling 'center-line' for each non-empty line.
(center-line-mode): New minor mode.
* etc/NEWS: Document the new minor mode.
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