method to look up families of i18n keys

Some messages are programmatically generated according to sets of reference strings (e.g. score model names like 'BLOSUM62'). these reference strings are arbitrary, and may even contain strings defined by the user, for which no translation may be available.

A new MessageManager.getString method is needed to look up specific instances of these kind of 'compound keys', and if no string is found, return the reference string unmodified so it can be shown to the user.