Implemented "midimessage" event dispatching on noteOn/noteOff

[kyr0]

When the visual keyboard of the synth gets clicked, a midi message event gets created and dispatched on window.onmidimessage (on window because there is no prototype to dispatch on currently). This allows third party software to use the virtual keyboard clicks as MIDI input. This impl. is using the data structure of the WebMIDIApi shim to be inter-operable and standard conform. Thank you @cwilso — WebMIDIApi is awesome!