movePage method
Implementation
Future<bool> movePage(int from, int to) async {
final List<UPdfRef> refs = await flattenPageTree();
if (from < 0 || from >= refs.length || to < 0 || to >= refs.length) return false;
final UPdfRef moved = refs.removeAt(from);
refs.insert(to, moved);
return setPageOrder(refs);
}