Allow Highcharts views to render large datasets

Problem/Motivation

Using views_plugin_style::render() to get labels from $view->style_plugin->rendered_fields is very taxing on 10000+ rows.

Proposed resolution

Get labeled field results of a view, as an efficient alternative to render(), and hijack render() with highcharts_views_plugin_style_highcharts::render().

API changes

Using this method, views theme templates will no longer work on Highchart views. However, that is probably desired anyway… see #1561694-6: Highcharts views theme functions and templates.

We already have a themeable function - theme_highcharts_chart() - and individual Highcharts-related theme concerns can already be addressed on individual charts using the Highcharts library options API. Ultimately, for Highchart views display styles, we only want the data from Views - and we delegate all rendering to Highcharts. Hopefully this bypassing of Views standard rendering is worth the huge performance boost we will get.