Added a propertysheet so that the display of default page in a section can be turned off. This is useful for example to define translatable content through web pages in relation with a web section, although the web section should display a list of items. (ex. news, press, products in a shop, etc.)

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@22294 20353a03-c40f-0410-a6d1-a30d3c3de9de