[Translation] added translation domain cascading functionnality

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | symfony/symfony-docs#2825

With this PR, you can optionally pass an array of domains instead of a simple string to `Translator::trans()` or `Translator::transChoice`, allowing for multi-level fallbacks.

Added new tests specifics to this feature.