Display required permissions and warning when editing a page requiring specific permissions

See http://markmail.org/message/2666mufrbvqskvnq

The idea is:

• When a page has an xobject of type XWiki.RequiredRightClass
• When a user is editing that page
• Display the required permissions in the Panel on the top right (similar to the included pages that we display)
• Display a warning when the user editing doesn't have those permissions. This could be a modal popup for example. We could also prevent the user from editing the page in that case if the required permission is PR and the user doesn't have PR (in order so that he cannot break the page when saving).