Reviewed by Xan Lopez.

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

        Implements the AtkSelection interface and enables the corresponding
        (and expected) object:selection-changed event.

        * accessibility/gtk/AccessibilityObjectWrapperAtk.cpp:
        (optionFromList):
        (optionFromSelection):
        (atk_selection_interface_init):
        (webkit_accessible_selection_add_selection):
        (webkit_accessible_selection_clear_selection):
        (webkit_accessible_selection_ref_selection):
        (webkit_accessible_selection_get_selection_count):
        (webkit_accessible_selection_is_child_selected):
        (webkit_accessible_selection_remove_selection):
        (webkit_accessible_selection_select_all_selection):
        (GetAtkInterfaceTypeFromWAIType):
        * accessibility/gtk/AXObjectCacheAtk.cpp:
        (AXObjectCache::postPlatformNotification):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@50282 268f45cc-cd09-0410-ab3c-d52691b4dbfc