Provide a reverse legend option for the highcharts

I'm not sure if the `#access` part is relevant? I can't find any mention of `highcharts_settings` anywhere in code apart from that #access bit.

I'm removing the #access and re-uploading patch as it seems completely irrelevant here.

Thank you for the patch but if this config is being added only to be used for highcharts.

I think it should go under highcharts plugin configuration instead of the base settings element. To do this I think we can add a new reversed properties under legend in default configurations and then proceed to add the form element in the build configuration form of the plugin. The schema should also need to be updated for the new configuration property.