Help: Fixed that external help window didn't get focus

When opening examples. Testable by pressing F1 or trying to scroll in
the viewer with keyboard directly after it opened.

Change-Id: I6a2d4535daf38936140e1d1488cb78fb2e9019e3
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>

src/plugins/help/helpplugin.cpp

@@ -555,6 +555,7 @@ HelpViewer *HelpPlugin::externalHelpViewer()
     if (!m_externalWindowState.isNull())
         m_externalWindow->setGeometry(m_externalWindowState);
     m_externalWindow->show();
+    m_externalWindow->setFocus();
     return m_externalWindow->currentViewer();
 }