Melody events (#391)

* support for custom playTone

* added music.onEvent, music.setPlayTone

* updated docs

* updated sample

docs/reference/music/on-event.md

@@ -0,0 +1,51 @@
+# On Event
+
+Raises events for melodies or music events.
+
+```sig
+music.onEvent(MusicEvent.MelodyNotePlayed, () => {})
+```
+
+### Parameters
+
+* ``value`` the kind of event
+* ``handler`` the code to run when the event is raised.
+
+### Example
+
+This example prints all the events to the serial output.
+
+```blocks
+music.onEvent(MusicEvent.MelodyRepeated, () => {
+    serial.writeLine("melody repeated")
+})
+music.onEvent(MusicEvent.MelodyEnded, () => {
+    serial.writeLine("melody ended")
+})
+music.onEvent(MusicEvent.MelodyStarted, () => {
+    serial.writeLine("melody started")
+})
+music.onEvent(MusicEvent.MelodyRepeated, () => {
+    serial.writeLine("background melody repeated")
+})
+music.onEvent(MusicEvent.BackgroundMelodyStarted, () => {
+    serial.writeLine("background started")
+})
+music.onEvent(MusicEvent.BackgroundMelodyEnded, () => {
+    serial.writeLine("background ended")
+})
+music.onEvent(MusicEvent.BackgroundMelodyPaused, () => {
+    serial.writeLine("background paused")
+})
+music.onEvent(MusicEvent.BackgroundMelodyResumed, () => {
+    serial.writeLine("background resumed")
+})
+music.onEvent(MusicEvent.BackgroundMelodyRepeated, () => {
+    serial.writeLine("background repeated")
+})
+input.onButtonPressed(Button.A, () => {
+    music.beginMelody(music.builtInMelody(Melodies.BaDing), MelodyOptions.Once)
+})
+music.setTempo(100)
+music.beginMelody(music.builtInMelody(Melodies.Ringtone), MelodyOptions.ForeverInBackground)
+```