Added some validation to DefaultModelRegistry, to verify that an element is not changed after it has been closed.

Currently does not validate the removal of links.

Also verify that a mutation added for a subject will fire, and will not be ignored due to the subject being in a later lifecycle state.
For now, ignore elements added to a DomainObjectContainer after the container has been closed - eg placeholder tasks and tasks added using an old style rule.
Some test tweaks.
Updated spec to add another story for dealing properly with placeholder tasks.