Generic solution available for entity editing overwrite avoidance

The NodeChangedConstraint and NodeChangedConstraintValidator that were previously only available for nodes have now been generalized for all entities. This constraint validator is useful to validate entities to avoid cross-edit overwrites. When user A and user B open an entity for editing and user B saves before user A, user A should get a validation error for editing an old version of the entity.

The constraint and validator are available generically. To use it, do the following:

1. Add a changed timestamp to your entity base fields (in your entity database schema for example) and define it with the baseFieldDefinitions() method, while assigning the EntityChanged constraint (excerpt from Node.php):
   

       $properties['changed'] = array(
         'label' => t('Changed'),
         'description' => t('The time that the node was last edited.'),
         'type' => 'integer_field',
         'property_constraints' => array(
           'value' => array('EntityChanged' => array()),
         ),
       );
   

2. Make your entity class implement EntityChangedInterface, so that it provides the changed timestamp via a getChangedTime() method. See NodeInterface.php and Node.php in core for example.
3. By associating the constraint with the base field and implementing the interface, the constraint will be able to retrieve the changed timestamps on the edited and saved entities and compare them for detecting editing conflicts. To get the validation messages, use:
   

       $violations = $entity->validate();
   

   If your copy of the entity has an older changed timestamp than the one saved in the system, you'll get a validation error in the returned array. In practice you'll get data like:

     // This will be 'changed.0.value'.
     $property = $violations[0]->getPropertyPath();
     // This will be t('The content has either been modified by another user, or you have already submitted modifications. As a result, your changes cannot be saved.');
     $message = $violations[0]->getMessage();