Reviewed by Adam.
WebCore:
        More drag and drop migration, now the entirety of the
        drag initiation logic has been rendered platform independent
        This has required a number of new interfaces, and a reasonable
        amount of logic migration.

        As a side effect, this patch also fixes rdar://problem/4945341

        There are some basic Qt stubs that should stop the build from failing,
        however the absence of ClipboardQt means any attempt to initiate a drag 
        may cause a crash.

        * WebCore.exp:
           Exporting new symbols

        * WebCore.xcodeproj/project.pbxproj:
           New files

        * dom/Clipboard.cpp:
        (WebCore::Clipboard::canSaveAsWebArchive):
          Migrated from WebKit

        * dom/Clipboard.h:
          Added more methods to allow Clipboard to be used as a 
          platform independent container for drag and drop

        * page/DragClient.h:
        (WebCore::DragClie...