Changes

There is a known issue with ImageIO plug-ins and containers (like a web/servlet container).
Deploying plug-ins as part of the web application is not well supported by ImageIO.
The ImageIO registry that keeps track of registered plug-ins is in effect JVM global
(it is actually a registry per application context, however, there is usually only a single application context).
It is then require to provide an initializer to register and unregister
all the ImageIO plugins for each deployment/undeployment of the
Silverpeas application.

Fix #14939