Expose parent metadatas through ListMetadata

Refs https://redmine.teklia.com/issues/1981 \ Needed for callico.

We need an optional boolean parameter on ListElementMetaData named load_parents, defaults to False.

When this parameter is enabled, all the metadatas from parents (any path) are loaded amongst the metadatas directly set on the element.

There is no need to identify the source of the metada (if it's set on a parent or directly on the element).