This way we can reuse this test in projects code

DeliveryLine_init sets a incremental reference for each delivery line, so this
reference cannot be used for searching

A set setter keeps the default value, so in a sense the order is preserved.

Ideally the signature of accessor should be more strict

Also, factorise & simplify code.
Also, follow coding style.
Also, follow naming conventions.
Also, avoid calling dict() for static key sets.
Also, add comments about bad code kept for backward compatibility.
Also, add comments about incorrect grammar - also kept for backward
compatibility.

Add constraints to EventConstraint property_sheet to check if destination, resource and source are set and move TALES constraint to a new property sheet that is specific to Mail Messages. By default both are not enabled.

Add property sheet to check if the event destination have email address defined. By default, this constraint is disabled

This way if we select a notification message we can leave the title empty and use the title from notification message (which supports substitution)