When a new value for the HelpText property introduced in

    commit 8f1c7015
    Author: Mike Gorse <mgorse@suse.com>
    Date:   Wed Jan 3 16:52:31 2024 -0600

        Add HelpText property and corresponding functions in atk and libatspi.

is set in `atk_object_set_help_text`, send an
"object:property-change:accessible-help-text" event,
so ATs are notified and can react to such changes.

As is the case for the accessible name and description,
don't notify when the help text is initially set and
don't accept explicitly setting null values, see also [1].

[1] https://bugzilla.gnome.org/show_bug.cgi?id=665870