Improve html rich text support

If contains only one text fragment with format, then it should be plain
text. And the format should be considered as Cell format

examples/xlsx/richtext/main.cpp

@@ -27,6 +27,9 @@ int main()
 
     xlsx.workbook()->setHtmlToRichStringEnabled(true);
     xlsx.write("B4", "<b>Hello</b> <font color=\"red\">Qt</font> <i>Xlsx</i>");
+
+    xlsx.write("B6", "<font color=\"red\"><b><u><i>Qt Xlsx</i></u></b></font>");
+
     //![1]
 
     //![2]

src/xlsx/xlsxworksheet.cpp

@@ -621,6 +621,8 @@ int Worksheet::writeString(int row, int column, const RichString &value, const F
 
     d->sharedStrings()->addSharedString(value);
     Format fmt = format.isValid() ? format : d->cellFormat(row, column);
+    if (value.fragmentCount() == 1 && value.fragmentFormat(0).isValid())
+        fmt.mergeFormat(value.fragmentFormat(0));
     d->workbook->styles()->addXfFormat(fmt);
     QSharedPointer<Cell> cell = QSharedPointer<Cell>(new Cell(value.toPlainString(), Cell::String, fmt, this));
     cell->d_ptr->richString = value;