Code refactor: numFmt part of QXlsx::Format

src/xlsx/xlsxstyles_p.h

@@ -40,13 +40,20 @@ class StylesTest;
 namespace QXlsx {
 
 class Format;
-struct XlsxFormatNumberData;
 struct XlsxFormatFontData;
 struct XlsxFormatFillData;
 struct XlsxFormatBorderData;
 class XmlStreamWriter;
 class XmlStreamReader;
 
+struct XlsxFormatNumberData
+{
+    XlsxFormatNumberData() : formatIndex(0) {}
+
+    int formatIndex;
+    QString formatString;
+};
+
 class XLSX_AUTOTEST_EXPORT Styles
 {
 public: