*Really* implement "in column group" insertion mode.
*Really* implement "in column group" insertion mode.

svn path=/trunk/hubbub/; revision=4441

Achal-Aggarwal/hibbub,Achal-Aggarwal/hibbub,prepare/netsurf_libhubbub,servo/libhubbub,prepare/netsurf_libhubbub,servo/libhubbub,adamwight/wmf-parsoid-libhubbub,adamwight/wmf-parsoid-libhubbub,adamwight/wmf-parsoid-libhubbub,prepare/netsurf_libhubbub,Achal-Aggarwal/hibbub,prepare/netsurf_libhubbub,servo/libhubbub,prepare/netsurf_libhubbub,servo/libhubbub,adamwight/wmf-parsoid-libhubbub,Achal-Aggarwal/hibbub,servo/libhubbub,prepare/netsurf_libhubbub,adamwight/wmf-parsoid-libhubbub