[2.0.0] GLSP client breaks when opening multiple diagrams

[tortmayr]

Opening multiple diagrams (associated with the same GLSP client) breaks the GLSPClient.onActionMessage notification and only the last opened diagram will still receive messages from the server.
The issue is that we directly hook to the connection.OnNotification(..) method from vscode-jsonrpc which infact can only handle one listener. Registering a new listener by opening a new diagram will override any existing listener.
To fix this we should introduce an intermediate event emitter that is hooked to the connections onNotification methods. Any listeners registered with GLSPClient.onActionMessage will then be registered on that emitter.