XWIKI-10252: Add and/or edit objects in a page

* Avoid a NullPointerException
XWIKI-10252: Add and/or edit objects in a page

* Add some predefined string identifiers
XWIKI-10252: Add and/or edit objects in a page

* Remove an empty comment
XWIKI-10252: Add and/or edit objects in a page

* Make the object policy keywords 'final'