Fix merge of DocumentsList descriptor

The current merge does not take into account the events, so it is not possible to override them with a new contribution.

2 merge behaviors are possible:

• the events from the new descriptor completely override the events from the old one
• the events from the new descriptor are merged with the events from the old one

Each have pros and cons. As part of a fix which will not introduce new attribute, the first one has the advantage not to be stuck if one needs to remove events.