Also disallow deletion of only non-permanent page.

#736

IDs can and will be generated by the element constructors.

Disallow state variables without alias.

They are registered when being created.

#713

#716

This method is the same for all elements and can be implemented in the 
parent.

#703

This may happen when changing the Clote document property.

Also move cloze-code into Cloze and no longer have it in a general 
purpose service.

This is done within the element's logic.

The default value of the child class overrides the value set by the 
parent constructor.