Use naming convention for event listener names

[manzt]

There is no standardization of emitted event names. I'd prefer an <action>:<emitted> convention. E.g.,

let el = document.querySelector('#root');
let viewer = vizarr.createViewer(el);

viewer.on(
--  "viewStateChange",
++  "change:viewState",
  () => {
  /* ... */
});