Reviewed by Chris Fleizach.

        [GTK] Improve accessibility of focusable lists
        https://bugs.webkit.org/show_bug.cgi?id=25679

        Emit the 'selected' and 'focused' events as needed.

        This is the last bit of a series of patches to fix bug 25679,
        which just ensures that the right signals are emmited whenever a
        selection inside a listbox object changes, that is, the signals
        'state-changed::selected', 'state-changed::focused' and
        'focus-event', along with the right detail for each of them to
        report if the focus/selection has been activated or not.

        * accessibility/gtk/AXObjectCacheAtk.cpp:
        (WebCore::notifyChildrenSelectionChange): New, takes care of
        emitting all the needed signals when children selection has
        changed, both from the point of view of the container and the
        selected/unselected items inside of it. It currently supports
        listboxes (html 'select' controls) only.
        (WebCore::AXObjectCache::postPlatformNotification): Replaced some
        old code with a simple call to notifyChildrenSelectionChange().

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@71710 268f45cc-cd09-0410-ab3c-d52691b4dbfc