New UI for setting required rights

A new UI for setting the required rights of a page has been added. Required rights are the rights that a page requires for executing script macros or registering a UI extension. This new UI allows selecting if required rights shall be enforced, and allows specifying which rights shall be enforced. When required rights are enforced, the page can only use the specified rights and only users with those rights can edit the page. This avoids that, for example, a script macro added by a user without script right is executed when a user with script right saves the page, or that users accidentally break a page when they edit it without having the required rights.