[Translation] [Xliff] Support for <note>

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10550
| License       | MIT
| Doc PR        | n/a

The purpose of this PR, is to avoid that the automated translation update (`app/console translation:update`) remove the `note` tags from XLIFF files.


Not sure if using the metadata bag is a good thing or if I should have add a "note" property in MessageCatalogue. What do you think ?
The parenthesis around the assignment should be removed (same below).