When splitting, keep named page destinations that are part of target document(s)

Keep named destinations. The current code just ignores them. I wrote some 40 lines that would create a name tree in the destination document, but this didn't work because the destination name gets modified when retrieved as a string. So I just keep the actual destination and forget the name, which is a single code line. It's a new document anyway and the average user expectation is that the links "just work".