Refactoring to put browser specific code in a new class (#3718)

Refactoring to put browser specific code in a new class, BrowserPlatform.
BrowserPlatform implements IPlatform. Chart.Platform is the constructor for the platform object that is attached to the chart instance.

Plugins are notified about the event using the `onEvent` call. The legend plugin was converted to use onEvent instead of the older private `handleEvent` method.
Wrote test to check that plugins are notified about events