[meta] Drupal 8 port of Simple hierarchical select

Tracking ticket for the work on SHS 8.x-1.x.

What's done?

Form display

Displaying term reference fields as hierachical widgets is working and acceptable fast (unfortunately not as fast as in Drupal 7 using the High-performance JavaScript callback handler but ... you know, fast).
Multiple-value handling has changed to an all-Js-solution which is a little faster and more comfortable to handle (at least for me).

Formatter

The default formatter (including linking each item to its entity) is also working.

Views filters

Exposed filters for taxonomy term references may be rendered as Simple hierarchical select widgets. This works with or without the "Depth" option and even with multiple selections.

Chosen integration

This is done in a sub-module. See #2712123: Add support for chosen

What's missing?

Extended configuration

Adding new terms had been added as configuration option but does not do anything yet. This will be implemented until 8.x-1.0.
See #2712115: Add functionality to add new terms.

Documentation

Because of the major rewrite and extensive usage of Backbone.js a developer documentation is needed on how to alter and override things in Simple hierarchical select.. shs.api.php is a good start, but example modules would be way better.
See #2712117: Example module about how to alter SHS.