If new_child is already in the tree ensure its parent is not read only
If new_child is already in the tree ensure its parent is not read only

svn path=/trunk/dom/; revision=3556

dunkelstern/libdom,Achal-Aggarwal/libdom,Achal-Aggarwal/libdom,prepare/netsurf_libdom,Achal-Aggarwal/libdom,prepare/netsurf_libdom,dunkelstern/libdom,prepare/netsurf_libdom,dunkelstern/libdom,Achal-Aggarwal/libdom,Achal-Aggarwal/libdom,prepare/netsurf_libdom,dunkelstern/libdom,prepare/netsurf_libdom,dunkelstern/libdom,prepare/netsurf_libdom,Achal-Aggarwal/libdom,dunkelstern/libdom