Reviewed by Gustavo Noronha.

        Implement AtkObject::focus-event and
        AtkObject::state-changed:focused signal emission.

        Adds a new handleFocusedUIElementChangedWithRenderers, only in the
        GTK port, called at the same point than
        handleFocusedUIElementChanged but with two parameters, the old and
        the newly focused RenderObjects. We need this, since the ATK
        signals require us to pass the objects involved in the focus
        change as arguments.

        * accessibility/AXObjectCache.h:
        * accessibility/gtk/AXObjectCacheAtk.cpp:
        (WebCore::AXObjectCache::handleFocusedUIElementChangedWithRenderers):
        * dom/Document.cpp:
        (WebCore::Document::setFocusedNode):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@43920 268f45cc-cd09-0410-ab3c-d52691b4dbfc