3.0 entity context fixes

With this set of changes the Marshaller and table will mark the entities thy create or save with information about the repository they are bound to. This makes it possible to use plain entities in the form helper as long as you create them using the marshaller.

```php
$entity = $this->Articles->newEntity([]);
debug($entity->source()); // outputs ['alias' => 'Articles', 'className' => 'Cake\ORM\Table'];
....
$this->Form->create($entity); // Works!
```