Warn users if tutor misconfigured

[tomcucinotta]

Occasionally, it happens that PianoTutor starts with an empty configuration (LEDs per meter equal to 0), with the effect that only one LED is turned on/off, so the system seems completely broken. It is sufficient to enter that LEDs/m value in LEDs set-up pane to restore the functionality, however the tutor LED set-up pane is not immediately visible. Therefore, it would be good if, on turning on the PianoTutor function, the system could display a warning to the user if it detects that there is no LEDs/m value configured in the settings, and let the user readily fix that.