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
Prior to this commit, translations were built in the serializers. This
could not work as a single translation can be used for different source
messages having different placeholder content.
Serializers do not try to replace the placeholders any more.
Placeholders are replaced by the translation bundle and the source
message is given as parameter so that the content of the placeholders is
taken into account.
Also XMB ids are now independent of the expression which is replaced by
a placeholder in the extracted file.
fixes#12512
