Rename `Chart.pluginService` to `Chart.plugins` (so move the old Chart.plugins array as a private member of the service), and rename `notifyPlugins` to `notify` for consistency with other service methods.
Data controllers should now rarely implement addElements and addElementAndReset but instead should define dataElementType (and optionally datasetElementType). Also remove some dead code (e.g. numBars, colorForNewElement, etc.).