Added a property to store the ID of a page used to configure the current Web Section of Web Site layout. This will help the process of making all widgets generic. The cleanest approach would be obviously to rely only on the gadget API but it is less convenient than flexible properties when it comes to pure page templates.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@19946 20353a03-c40f-0410-a6d1-a30d3c3de9de