checkConnectionIsAlive: fire event

Problem/Motivation

With #3161206: add submodule auth_user_alfresco: the checkConnectionIsAlive method will log the user out if the Alfresco's user token has expired.

Proposed resolution

I think the best thing to do is to fire an event, putting the check result in the event.

So other modules can react in consequences. For example, the base module could provide a settings form with options on what to do. Log the user out or redirect to the homepage, etc. Some websites will not want the user to be logged out.

Remaining tasks

Implements a patch.

User interface changes

API changes

Data model changes

Release notes snippet