Migrate support

This is an approach that adds a new migrate destination field to any entity where to specify the entityqueue/s where the entity has to be added.

This patch corrects the previous way to get the values of the entityqueues names from $entity instead of from $row.
In this way it guarantees that when multiples values are received they arrive in an array without any separator.

The previous patch contains only the interdiff, this one is the right one.

The patch looks ok to me as well. Here's a few things that could be improved:

1. +++ b/entityqueue.migrate.inc
   @@ -0,0 +1,87 @@
   + * DestinationHandler.
   

   This description could be expanded with a few more words :)

2. +++ b/entityqueue.migrate.inc
   @@ -0,0 +1,87 @@
   +   * Overrides fields().
   ...
   +   * Overrides complete().
   +   *
   +   * @param object $entity
   +   *   The Drupal entity.
   +   * @param object $row
   +   *   The row being migrated.
   

   These can be simply {@inheritdoc}.

3. +++ b/entityqueue.migrate.inc
   @@ -0,0 +1,87 @@
   +      $migrate_entityqueue = $entity->entityqueue;
   

   We can change this to:

   $migrate_entityqueue = (array) $entity->entityqueue;
   

   and drop the conversion below.

4. +++ b/entityqueue.migrate.inc
   @@ -0,0 +1,87 @@
   +
   ...
   +
   

   Unneeded empty lines in the complete() method.

5. +++ b/entityqueue.migrate.inc
   @@ -0,0 +1,87 @@
   +    $migration = Migration::currentMigration();
   +    $destination = $migration->getDestination();
   +    $entity_type = $destination->getEntityType();
   

   It looks like we're not using $migration or $destination below, so we can inline everything to:

   $entity_type = Migration::currentMigration()->getDestination()->getEntityType();
   

Thanks, looks much better :)

+++ b/entityqueue.migrate.inc
@@ -0,0 +1,77 @@
+ * Doesn't support rollback, when you rollback the entities with this field the
+ * number of items in the entityqueue is not updated.

I wonder why this is the case. We can not support rollback at all or it's just not part of this implementation because you didn't need it?

Removed the files[] declaration because migrate already detects the file.

Are you sure it detects the file even when the hook_migrate_api() implementation is in that file? I kind of doubt that.. :) It would make more sense to move the hook at the bottom of entityqueue.module.

Oh, I overlooked the fact that our filename ends in .migrate.inc, I thought it was named after the class name like we do in Drupal 8. So, yep, I think we're good here.

Setting back to RTBC to give some time to @jojonaloha if he wants to review this as well.

I haven't tested this, but the only thing I see is I would change $key = 'eq_' . $entity_type; to $key = _entityqueue_get_target_field_name($entity_type);

Closing issues for the 7.x version, which is not supported anymore.