Twig will always use gettext, legacy will use legacy. Twig does not
work with legacy so new ui would break if you would not also set this
option. Instead, just switch the system in use based on the usenewui
config variable.

This is because some templates, e.g. errror, expect to be able to
include them. This fixes e.g. the No State error page.

This is useful when testing.

*    Updated the phpDoc comments to match the code
*    Added return statements
*    Added return values

This introduces the following:

- The use of Request objects to handle request data to controllers.
- The use of Response objects to model responses that should be sent to the browser.
- The use of "controllers" that are responsible for translating a request into a response.
- The possibility to define your own URLs on each module by specifying them, together with their controllers, in a "routes.yaml" file in the root of a module.
- The new UI is completely separated from the old, so "usenewui" must be set to "true" in the configuration.
- Twigified templates are not used unless we're using the new UI, or the twig template is part of a theme.

This seemed the logical place to do it, because it's essentially
a presentation thing (so in Template, not Locale). There's likely
something to be said for sorting by key aswell as sorting by
translated language name. Picked key for now, seems most predictable.

setupTwigTemplatepaths() cannot return false

This seemed the logical place to do it, because it's essentially
a presentation thing (so in Template, not Locale). There's likely
something to be said for sorting by key aswell as sorting by
translated language name. Picked key for now, seems most predictable.

This allows us to load templates from modules inside a twig template itself (include, embed, etc), even when the main template is not in a module or in a different one.

This can be used by templates to load resources in different ways, either optimized for the developer or for a production environment.

This allows us to use this new filter to translate strings from a given array of translations, where every translation is indexed by its ISO 639 code. A new configuration option ('language' -> 'priorities') is available too to control the alternative languages that can be used instead of a given language, when the latter is not found. The filter returns null when no suitable translation is found, so that it can be combined with "default()" to set a default translation for a given string.

This resolves #695.

Otherwise, a theme would not be able to include/embed/extend its own templates.

This new interface allows themes to define a class that can be hooked at certain specific points of template initialization/handling, so that they can do stuff like automatically adding variables for all templates, or adding twig extensions. This classes must implement the new TemplateControllerInterface, and be specified in the "theme.controller" configuration option. This way, we avoid the performance hit if we use traditional hooks, and we also avoid hooks from other modules causing trouble.

For now, the interface offers two entry points: setUpTwig(), which allows managing the twig environment after initialization (e.g. to add an extension or define filters); and display(), which offers all the data passed to the template, and allows adding or modifying it.

This makes sense as those should be static values available to every template.

Additionally, add a "templateId" variable that we can use for templates to identify themselves.

Make sure if we are using a theme, its module is added as a valid domain where we can look for translations.