Fix the 'Event' dropdown when creating a retention event

The Event dropdown does not show any suggestion when creating a retention event:

How to reproduce:

• fire up a 10.10 Nuxeo instance with the nuxeo-retention addon
• log in the Web UI
• open the Retention drawer and click on Retention Event
• click on the input field of attribute Event to open the dropdown

Expected result: the dropdown open and suggest retention events as show in our documentation.
Actual result: the dropdown does not suggest any retention events.