Refactor Theme Registry

The OO approach will likely have to be in a module if we actually end up needing to fundamentally change how the theme registry service works. If so, this will likely need to be implemented via a custom module inside the theme (which is possible in 8.x, "unofficially").

Or, we simply keep hook_theme_registry_alter() and reduce it to what we actually will probably need: adding includes to each hook registered.

Also, there is now a new hook in 8.x that should be utilized for adding things like the context, icon, and icon_position variables: hook_template_preprocess_default_variables_alter().

Ok. I've fixed the theme registry altering stuff temporarily so templates and preprocessing will start working.

That being said, now it's just a huge task of converting all the theme functions and tpl.php files into html.twig files.

I'm leaving this open for now as this really should be OO and fixed properly before a release.

Oops

Now that the majority of the 8.x codebase is OO and preprocessing is discoverable via @BootstrapPreprocess annotation plugin, this should get another once over.