[Server] Refactor UmlDisposeClientSessionActionHandler to use a ClientSessionListener instead

[CamilleLetavernier]

UmlDisposeClientSessionActionHandler will only react to DisposeClientSessionAction explicitly sent by the Client to dispose the GLSP/ModelServer connection. If the client disconnects without sending this action (e.g. because the browser tab is closed), UmlDisposeClientSessionActionHandler will never be invoked.

We should refactor this class and implement ClientSessionListener#sessionClosed instead