This PR was merged into the 2.7 branch.

Discussion
----------

Twig decoupling from Templating

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #11748, #11876
| License       | MIT
| Doc PR        | n/a

The goal of this PR is to make Twig independent of the Symfony Templating system. With this PR, you can now use the Twig bundle without activating `framework.templating`. Also, even when registering the templating system in FrameworkBundle, the `php` engine is only registered when specified explicitly in `engines`.

First, the global variables has been decoupled from FrameworkBundle.

Then, the Twig bundle now tries to only rely on native Twig extensions and loaders and use the Templating sub-system only if `framework.templating` is enabled.

Commits
-------

18d4c41 [TwigBundle] added some tests
0d537c4 decoupled Twig from the Templating system
be5a208 decoupled global variables system in Twig from the Templating one

This PR was merged into the 2.7 branch.

Discussion
----------

[TwigBundle] Fix twig loader registered twice

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #20665
| License       | MIT
| Doc PR        | N/A

Generated code:

### Before

```php
    protected function getTwig_LoaderService()
    {
        $a = new \Twig_Loader_Filesystem(array(), $this->targetDirs[3]);

        $a->addPath(...);
        // ...

        $this->services['twig.loader'] = $instance = new \Twig_Loader_Chain();

        $instance->addLoader($a);
        $instance->addLoader($a);

        return $instance;
    }
```

### After

```php
    protected function getTwig_LoaderService()
    {
        $this->services['twig.loader'] = $instance = new \Twig_Loader_Filesystem(array(), $this->targetDirs[3]);

        $instance->addPath(...);
        // ...

        return $instance;
    }
```

~~Another solution is to simply create a private alias. But I don't know if we should care or not about the case people may rely on the fact both services exist as definition, and not as an alias, in a compiler pass.~~ (Has been preferred over of using a child definition)

For reference, this issue was introduced in #13354.

Commits
-------

2c81819 [TwigBundle] Fix twig loader registered twice