Move Java client and server classes to separate packages

[bernardnormier]

The Java demos currently show Client and Server classes in the same directory, in the default package. Because of that, we create client.jar and server.jar that contain the same classes: they are identical except for the MANIFEST file.

We should:

• move Client and client-related classes to its own package
• move Server and server-related classes to its own package
• for demos with other components such as plugins and services, move each component to its own package
• create a separate jar file for each component (client.jar, server.jar, service.jar, plugin.jar...)