[3.6] Improving relation get set interface

[rossriley]

This is based on feedback from #7497 that the interface for interacting with relations has become a bit complicated.

This is a small PR that adds a helper method to make it easier to associate content entities with an owner.

The public-facing API is straightforward, given either an entity, a collection or a string/id combination we can easily associate relations.

$repo = $app['storage']->getRepository('entries');
$entry = $app['query']->getContent('entries/1');
// Or direct from the repo
$entry = $repo->find(1);

// Option 1: Associate relations with this entity by name and ids
$entry->relation->associate('pages', [1,2,3]);

// Option 2: Associate relations via passing a single Entity
$page = $app['query']->getContent('pages/1');
$entry->relation->associate($page);

// Option 3: Associate a set of relations by passing a collection
$pages = $app['query']->getContent('pages', ['id'=> '1 || 2 | |3]']);
$entry->relation->associate($pages);


// Save
$repo->save($entry);

[Nitpick-CI]

The closing brace for the class must go on the next line after the body

[bobdenotter]

I like it! 👍