onclick handlers incompatible with Content-Security-Policy

[minusdavid]

Hi folks,

I'm working on implementing a Content-Security-Policy on the Koha project (https://github.com/Koha-Community/Koha) and it looks like we're using this MIDI player.

When you use a Content-Security-Policy to prevent inline Javascript from running, inline event handlers like onclick won't work anymore.

We could replace the onclick handlers found around https://github.com/rism-digital/midi-player/blob/master/player/midiplayer.js#L386 with addEventListener() calls and all should work well.

I'm happy to look at writing a patch and sending in a pull request.

[minusdavid]

Pull request available at #32