Add visual MIDI activity indicator

Flash LED 0 white for 30ms on any MIDI input.
Visible without serial - helps diagnose whether MIDI
is arriving when connected to iPad with Loopy Pro.

src/main.cpp

@@ -367,6 +367,7 @@ void setup() {
 }
 
 void loop() {
+    led_driver.update();
     controller.update();
 
     if (Serial.available()) {