[Translation] Added unescaping of ids in PoFileLoader

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Although it is not directly described in gettext docs, _msgid_ should be unescaped too. The other reason to unescape _msgid_ is symmetry between ```PoFileLoader``` and ```PoFileDumper```. The dumper escapes both _msgid_ and _msgstr_ values, but the loader unescapes only _msgstr_.