qt: Fix richtext detection hang issue on very old Qt versions

Alternative implementation. Thanks to @awoland for the original.
Fixes #3486.

src/qt/guiutil.cpp

@@ -361,11 +361,11 @@ bool ToolTipToRichTextFilter::eventFilter(QObject *obj, QEvent *evt)
     {
         QWidget *widget = static_cast<QWidget*>(obj);
         QString tooltip = widget->toolTip();
-        if(tooltip.size() > size_threshold && !tooltip.startsWith("<qt/>") && !Qt::mightBeRichText(tooltip))
+        if(tooltip.size() > size_threshold && !tooltip.startsWith("<qt") && !Qt::mightBeRichText(tooltip))
         {
-            // Prefix <qt/> to make sure Qt detects this as rich text
+            // Envelop with <qt></qt> to make sure Qt detects this as rich text
             // Escape the current message as HTML and replace \n by <br>
-            tooltip = "<qt/>" + HtmlEscape(tooltip, true);
+            tooltip = "<qt>" + HtmlEscape(tooltip, true) + "</qt>";
             widget->setToolTip(tooltip);
             return true;
         }