There are restrictions on the character set that can be used for xmb and xtb
placeholder names.
However because changing the placeholder names would change the message IDs it
is not possible to add those restrictions to the names used internally. Then we
have to map internal name to public names when generating an xmb file and back
when translating using an xtb file.
Note for implementors of `Serializer`:
- When writing a file, the implementor should take care of converting the
internal names to public names while visiting the message nodes - this is
required because the original nodes are needed to compute the message ID.
- When reading a file, the implementor does not need to take care of the mapping
back to internal names as this is handled in the `I18nToHtmlVisitor` used by the
`TranslationBundle`.
fixes b/34339636
d02eab498f