Instead of just setting one side of the relation to null in the through
list, remove the rows entirely.
Remove only the relations which match the filters that have already been
set on the list.
This is consistent with the way ManyManyList works.
Also some small tidy-up (removing an unnecessary line break and an
unused "use" statement)
This provides a mechanism for adjusting the behaviour of these
relations when building more complex data models.
For example the following example has a status field incorporates a
Status field into the relationship:
```php
function MyRelation() {
$rel = $this->getManyManyComponents(‘MyRelation’);
$rel = $rel->filter(‘Status’, ‘Active’);
$rel->addCallbacks()->add(function ($relation, $item, $extra) {
$item->Status = ‘Active’;
$item->write();
});
}
```
Introduces a new library dependency: http://github.com/sminnee/callbacklist
API Add DataObject::setComponent()
API Support unary components as getter and setter fields
API ManyManyList::add() now supports unsaved records
ENHANCEMENT Animal farm
