[Translation] XLIFF 2.0 read support

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11853
| License       | MIT

- added XLIFF v2.0 support in `XliffFileLoader`.
- detects XLIFF version automatically from `version` or `xmlns` attributes.
- throw exception if XLIFF version does not exist.

Matches XLIFF 1.2 support, except that notes are not loaded now.
This is because they're not tied to a single translation (segment) anymore, but to a group of them (unit) or a whole file.