WebSocket connection closes if not kept alive

[fnunnari]

As by definition of the web socket protocol, connection can die if not kept alive.
We must implement a PING/PONG system with automatic send/recv every e.g., 15 seconds.
For the moment, shortcut is to press "Update clients" every now and then.

This implementation need refactoring:

• Use the Timer class to send a PING every now and then: https://pythonpyqt.com/qtimer/
• Move the self.ws.recv() call in a separate thread, and discriminate if it is receiving a PONG, or the clients list. See here for an example: https://stackoverflow.com/questions/9957195/updating-gui-elements-in-multithreaded-pyqt