server: add listener API for new clients

Using display object, Emit a signal if a new client is created.

In the server-side, we can get the destroy event of a client,
But there is no way to get the created event of it.
Of course, we can get the client object from the global registry
binding callbacks.
But it can be called several times with same client object.
And even if A client creates display object,
(so there is a connection), The server could not know that.
There could be more use-cases not only for this.

Giulio: a test is added for the new functionality

Signed-off-by: Sung-jae Park <nicesj@nicesj.com>
Signed-off-by: Giulio Camuffo <giulio.camuffo@kdab.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>