Add a loading indicator to nuxeo-selectivity when a request is run in the background

When a click is done inside a suggestion widget, a message is displayed:

• either a "Loading..." message while all the entries are fetched
• either a message to indicate the number of characters which must be entered to begin the search

After that, when some characters are entered to narrow the results, there is no indicator where a request is run in the background.

The ticket is to request an improvement to display an indicator when there is a background request.