eww.el: Use `track-changes`

eww.el: Use `track-changes`

* lisp/net/eww.el: Require `track-changes`.
(eww-display-document): Don't `inhibit-modification-hooks` any more.
Use `track-changes-register` *at the end* instead.
(eww-mode): Don't use `after-change-functions` any more.
(eww--track-changes): New function.
(eww--process-text-input): Rename from `eww-process-text-input`.
Try and be more careful: don't presume `point` is near
the modification.  Check for a form both at BEG and at END.
Don't rely on `:start/:end` pointing to the right places.
Use `:length` instead and shrink the field back to its original length
when possible.
(eww-size-text-inputs): Set `:length` rather than `:start/:end`.

(cherry picked from commit 168b67b1eeb2e2b79d2e5f3712d8ebfda31fd753)