Notify users when Live Plot disconnected

[slominskir]

Currently the Live Plot continues to scroll using the last known value in the face of a network disconnect and no notification is given. This is especially problematic with signals that do not update frequently and long periods of time with the same value is the norm. The web browser developer console does log errors in this scenario as the epics2web lib does notice the disconnect (using Web Socket liveness ping). Some form of notification, even a subtle disconnect icon/image in the subtitle would be an improvement. If a clock is added to the subtitle (#13) turning it red and freezing it could work as well.