Notifications and refactoring

New features:

• system notification (using the notify-send command), with a cooldown of 20s;
• the :info command shows which users are currently connected to the server. This is triggered automatically for a client after it has just connected.
• connection polling: the server polls its connections and removes ones that are already closed.
• connected users are notified whenever someone disconnects or when a new user joins the server.

In this release, the server and client code were merged into a peer file, since they already had a lot in common.