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.

#700

#700

It suddenly works without that. No idea why.

#702

This should allow users to notice, when errors happen and possibly 
report them.

It now uses the page index the menu (that was used to delete) belongs 
to, instead of the selected page index.