Reviewed by Beth.

        More WebCore context menu work (still not turned on, however).

        Split ContextMenuItem into its own files and make it a class.

        * WebCore.exp: Updated symbols.
        * WebCore.xcodeproj/project.pbxproj: Added new ContextMenuItem files.
        * platform/ContextMenu.cpp:
        (WebCore::ContextMenu::populate): Code cleanup.
        * platform/ContextMenu.h: Split ContextMenuItem into a separate file,
        removed redundant "Menu" part of platformMenuDescription(),
        setPlatformMenuDescription().
        (WebCore::ContextMenu::ContextMenu):
        (WebCore::ContextMenu::hitTestResult):
        (WebCore::ContextMenu::platformDescription):
        * platform/ContextMenuItem.h: Added. Made ContextMenuItem a
        full-fledged class.
        (WebCore::):
        (WebCore::ContextMenuItem::ContextMenuItem):
        (WebCore::ContextMenuItem::menu):
        (WebCore::ContextMenuItem::platformDescription):
        (WebCore::ContextMenuItem...