Store the downloadId in the book (and in the library).

src/book.cpp

@@ -123,6 +123,9 @@ void Book::updateFromXml(const pugi::xml_node& node, const std::string& baseDir)
   m_size = strtoull(ATTR("size"), 0, 0);
   m_favicon = base64_decode(ATTR("favicon"));
   m_faviconMimeType = ATTR("faviconMimeType");
+  try {
+    m_downloadId = ATTR("downloadId");
+  } catch(...) {}
 }
 #undef ATTR