Platform event API abstraction

Move base platform definition and logic in src/platform/platform.js and simplify the browser -> Chart.js event mapping by listing only different naming then fallback to the native type.

Replace `createEvent` by `add/removeEventListener` methods which dispatch Chart.js IEvent objects instead of native events. Move `add/removeResizeListener` implementation into the DOM platform which is now accessible via `platform.add/removeEventListener(chart, 'resize', listener)`.

Finally, remove `bindEvent` and `unbindEvent` from the helpers since the implementation is specific to the chart controller (and should be private).