Make mailto links when used inside the documentation

[ChangeLog][Qt Assistant][Make mailto links work again in Qt Assistant]

Change-Id: Iec15c27bbced7848c800193723399b4e4abc11b9
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/assistant/assistant/helpviewer.cpp

@@ -171,7 +171,7 @@ bool HelpViewer::launchWithExternalApp(const QUrl &url)
             actualTmpFile.close();
             return QDesktopServices::openUrl(QUrl(actualTmpFile.fileName()));
         }
-    } else if (url.scheme() == QLatin1String("http")) {
+    } else if (url.scheme() == QLatin1String("http") || url.scheme() == QLatin1String("mailto")) {
         return QDesktopServices::openUrl(url);
     }
     return false;