Provide a "heart beat"/"ping pong" mechanism for websocket connections

Provide a way to inform the client and server side of a ZK application about the current websocket connection reliability/availability.

E.g. a configurable PING/PONG mechanism with corresponding client side (e.g. zk.watch) and desktop wide server side events (similar to onClientInfo) will give the application the chance to inform the user or the serverside about unreliable/lost connections.

At the same time keeping the websocket alive, avoiding idle connection timeouts.

Alternative, idea:

provide a mechanism to send custom messages from both client and server side over the websocket to implement application specific behaviors.