Let the desktop system handle mailto links.

Task-number: QTCREATORBUG-4058

Change-Id: If48f8150bc05418c3ac0b9e132b84ed7836e08fa
Reviewed-by: Robert Löhning <robert.loehning@nokia.com>

src/plugins/help/helpviewer.cpp

@@ -145,6 +145,10 @@ bool HelpViewer::launchWithExternalApp(const QUrl &url)
                 return QDesktopServices::openUrl(QUrl(saver.fileName()));
         }
     }
+
+    if (url.scheme() == QLatin1String("mailto"))
+        return QDesktopServices::openUrl(url);
+
     return false;
 }