Define TranslatableString...

... Perhaps it should be in Internat.h (which is at the lowest level of the
dependency graph), but later ComponentInterface.h will need to include it, and
I don't want includes/ to depend on src/.

Though there is still a linkage dependency on src if
TranslatableString::Translation() is used.

src/Internat.cpp

@@ -290,3 +290,8 @@ std::vector< Identifier > Identifier::split( wxChar separator ) const
    auto strings = ::wxSplit( value, separator );
    return { strings.begin(), strings.end() };
 }
+
+const wxString &TranslatableString::Translation() const
+{
+   return wxGetTranslation(*this);
+}