This patch adds .IsTranslated(), .IsTranslatedN() and related functions
to following objects:
- Translation
- Domain
This makes it possible to detect whether a string is translatable or not
during runtime.
Allows stable round-tripping of PO(T) data across multiple message extraction passes.
Avoids canonicalisation of header keys, while preserving other semantics.
Refactored a bit too, so we can use interfaces to take Mo and Po files
added fixtures
found that the parser for Po files have a bug... but it works... so not touched