[Twig filter] Add 'desc' twig filter

[ctaverne]

Hi,

I think desc twig filter is very usefull and it could be easier to migrate from JMSTranslation when we are using it in all our code.

More details about desc filter here: http://jmsyst.com/bundles/JMSTranslationBundle/master/usage

[mvrhov]

The light version of this was merged yesterday

[mvrhov]

@Nyholm
I'm going to say, that this should stay open.. As this filter doesn't behave exactly as the one in jms bundle

The jms one shows the string from desc, if there is no translation in file.
It happens then that you forget to extract/update the translations from the templates. So maybe we should be better and somehow mark those strings. e.g prefixed with double !

//code generated by desc filter inside this component
echo twig_escape_filter(
    $this->env,
    $this->env->getExtension('Translation\Extractor\Twig\TranslationExtension')
        ->runDescFilter(
            $this->env->getExtension('Symfony\Bridge\Twig\Extension\TranslationExtension')
                ->trans("security.reset_password.question", [], "AccountBundle"),
            "Forgot password?"
        ),
    "html",
    null,
    true
);

//code generated by desc filter inside jmsbundle
echo twig_escape_filter(
    $this->env,
    ((($this->env->getExtension('Symfony\Bridge\Twig\Extension\TranslationExtension')
            ->trans(
                "security.login.reset_password.question",
                [],
                "AccountBundle"
            ) == "security.login.reset_password.question")) ? (twig_replace_filter(
        "Forgot password?",
        []
    )) : ($this->env->getExtension('Symfony\Bridge\Twig\Extension\TranslationExtension')
        ->trans("security.login.reset_password.question", [], "AccountBundle"))),
    "html",
    null,
    true
);