Support both transcriptions style

Unfortunately we still need to support both transcriptions "style" in this release:

• the old ones, with a zone and a source are displayed using a selection in the children tree
• the new ones, without zone not source, are simply attached to an element and displayed in the details

Closes #346 (closed) anyway.
We still need to remove the whole tree display of transcriptions, but after the release: #349 (closed)