Make chart's renderable alterable via charts hooks

In order to do this, we need each chart to have a machine name, so I created the #chart_id property for this purpose. Additionally, I found that a single hook was not enough, since you may want to alter the chart *before* the library abstraction (for using existing options across all libraries), but also *after* the abstraction in the event you want to modify a raw chart definition before the charting library gets ahold of it. This patch adds these two hooks, plus the general more-specific versions for per-chart functions.

Gave it another review and looks good. Committed and pushed.

1. Can I overwrite the chart style in a custom module?

Yes, though if you need to adjust the data format it may require more work than just changing the chart type.

2. Can I also combine two views to get an combo chart like this

Yes you could do that, but you don't need to do this any more because we now have this feature built-in as part of #2052429: Make multiple series charts easier to create through Views. Just make a chart, then add a second display that is a "Chart add-on" that uses the first chart as a parent. Then the two charts will lay over the top of one another when the parent chart is rendered.