Help: Implement a real external help window

With this patch it is just used with the context help "always show in
external window" option.

Change-Id: I4bc6ba8e0e241500f6970d4cb72f66ba02075ef5
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>

src/plugins/help/generalsettingspage.h

@@ -55,7 +55,6 @@ signals:
     void fontChanged();
     void startOptionChanged();
     void returnOnCloseChanged();
-    void contextHelpOptionChanged();
 
 private slots:
     void setCurrentPage();