Custom event which gets fired when a marker has been changed

### Feature Request
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It would be nice to add custom event which gets fired when a marker has been changed like [onchange](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/change_event),  [oninput](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/input_event). That is useful to save an alteration to a marker or something.