Before the rev 10880 fix in xml/simple-node.cpp, text updating when dragging gradient...

Before the rev 10880 fix in xml/simple-node.cpp, text updating when dragging gradient handle worked solely due to a bug: a fake attribute change was fired on style= though it did not in fact change. Now that these fake change events are eliminated, we need to explicitly update the style of the text's arena items in _modified, same as is done e.g. in sp_shape_modified.