[7.x] use getTouchedRelations when touching owners

[patrickomeara]

This allows overriding getTouchedRelations()

I want to update certain polymorphic parents.

Using the example from the documentation: if a comment is updated it should update post, but never video

class Comment extends Model
{
    protected $touches = ['commentable'];
    /**
     * Get the owning commentable model.
     */
    public function commentable()
    {
        return $this->morphTo();
    }

    public function getTouchedRelations()
    {
        return $this->commentable_type === 'video' ? [] : $this->touches;
    }
}

getTouchedRelations() isn't currently used in the framework anywhere, overriding it has no affect, it's simply a getter.

The alternative solution to my problem is to use setTouchedRelations() when the model is booted. However, overriding most get*() methods in the framework actually have an affect, whereas this one doesn't.

This is backwards compatible as the base getTouchedRelations() on HasRelationships simply returns $this->touches.