Add output list of items names to iterate in TimelineItemsGet

[JorgeHernandezibanezinbibo]

Right now the operator TimelineItemsGet only outputs the list of items instances, what is enough. However, if we want to iterate over a instance list, we can not provide a instance list to the Iterator operator.
Because this, a list of string with the names of the items is required to give that list to the Iterator and use the TimelineItemGet operator to get the specific item from the instance list latter.

[JorgeHernandezibanezinbibo]

Keep this task on hold. IF we decide to implement an Iterator instance operator in Shift this Issue won't be required anymore.