Record the collection content type in the StreamerElement and thus on file.

Store in the StreamerElement of the StreamerInfo of a class with a
CollectionProxy the name of the content type.
It stored in the StreamerElement's title at the beginning in the form:

  <ContentType>

We will be able to use this information to recreate a proper collection proxy
in the emulated case.
(cherry picked from the v5-34-00-patches branch
 commit 9509aab2)