Assign roles to categories to manage the visiblity

[Fasse]

We should allow to assign multiple roles to each category. These roles have the permission to view this category and all of their items. If no roles are assigned than all users can see that category.

With this we can manage to show profile fields only for some roles.
This feature could also be interesting in other modules with categories.
Within the event module we can handle the existing visible for with this new feature and use the visible for as a assignment for roles that are allowed to participate to an event.