Reviewed by Chris Fleizach.

        [GTK] Do not reference AccessibilityRenderObject from platform dependent code
        https://bugs.webkit.org/show_bug.cgi?id=57955

        Do not reference AccessibilityRenderObject from platform specific code

        Added new virtual functions to AccessibilityObject.h and removed
        explicit references to AccessibilityRenderObject from GTK code.

        * accessibility/AccessibilityObject.h:
        (WebCore::AccessibilityObject::renderer): New virtual method,
        returning 0 by default. Overriden by AccessibilityRenderObject.
        (WebCore::AccessibilityObject::correspondingLabelForControlElement):
        New virtual method, returning 0 by default. It complements the
        already present method correspondingControlForLabelElement().

        * accessibility/AccessibilityRenderObject.h:
        (WebCore::AccessibilityRenderObject::renderer): Made virtual.

        * accessibility/gtk/AXObjectCacheAtk.cpp:
        (WebCore::emitTextChanged): Reference  AccessibilityObject,
        instead of AccessibilityRenderObject.
        (WebCore::AXObjectCache::nodeTextChangePlatformNotification):
        Ditto.

        * accessibility/gtk/AccessibilityObjectWrapperAtk.cpp:
        (webkit_accessible_get_name): Do not reference
        AccessibilityRenderObject.
        (webkit_accessible_get_description): Ditto.
        (setAtkRelationSetFromCoreObject): Ditto.
        (optionFromList): Ditto.
        (optionFromSelection): Ditto.
        (webkit_accessible_selection_clear_selection): Ditto.
        (webkit_accessible_selection_get_selection_count): Ditto.
        (webkit_accessible_selection_select_all_selection): Ditto.
        (textForObject): Ditto.
        (webkit_accessible_text_get_text): Ditto.
        (getPangoLayoutForAtk): Ditto.
        (webkit_accessible_text_get_caret_offset): Ditto.
        (baselinePositionForRenderObject): Ditto.
        (getAttributeSetForAccessibilityObject): Ditto.
        (accessibilityObjectLength): Ditto.
        (getSelectionOffsetsForObject): Ditto.
        (webkit_accessible_text_set_selection): Ditto.
        (webkit_accessible_text_set_caret_offset): Ditto.
        (webkit_accessible_table_get_caption): Ditto.
        (getInterfaceMaskFromObject): Ditto.

        * accessibility/gtk/WebKitAccessibleHyperlink.cpp:
        (getRangeLengthForObject): Ditto.

        Removed unused renderer() method from AccessibilityMenuList.

        * accessibility/AccessibilityMenuList.h: Removed unused method.
        * accessibility/AccessibilityMenuList.cpp: Removed unused method.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@83064 268f45cc-cd09-0410-ab3c-d52691b4dbfc