Changes from pareview.sh, including removing unused code and fixing issues related to the implementation of ChartsInterface.

Created a ChartsInterface as a place to define constants. Also moved the API example module into the modules folder. And fixed code formatting in some places.

Responding to issue 2869541: Only display libraries that are enabled as options in the Charting Library options field.

Add an example module for people wanting to understand how to use the Charts module without making use of Views. Also fixes small issue with X-axis title in Highcharts.

This commit takes all sub-module-specific code out of the core charts module, making the module more extensible. It also improves the libraries.yml files and removes unused code from various parts of the site.

This reverts commit 31a47799.

Initial commit of the new 2.x branch of Charts. Full porting to D7, new Google Charts and Highcharts support. Expanded docs and API support.