Add watchdog to client / visu to talk to a logic engine

[ChristianMayer]

It would be great when the displayed visu has a watchdog that regularly sends a message to a logic engine that the visu / browser is alive.

So when a logic thinks that the visu should be running but these alive messages aren't coming it could e.g. restart the visu computer to fix e.g. a crashed browser.

Note: This might first need the implementation of a client identifier (e.g. part of the URI?), so that the logic engine can distinguish multiple visu PCs.