[Translation] added method to expose collected message

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #2435 
| License       | MIT
| Doc PR        | symfony/symfony-docs#3386
What I also find bad is that you can get all messages for all domains, but not for all locales.
I'd propose to simply remove the domains parameter. The logic that it implements is can simply be done by the user. And the method should only have a single responsibility, and that is not filtering domains.