[libvoikko] Recent changes in spelling suggestion code

[Harri Pitkänen]

I have made more changes to spelling suggestion part of libvoikko. It is now 
possible to actually use different backends for generating spelling 
suggestions.

Additionally dependencies between spell checking and spelling suggestions have 
been removed. For this to be possible I had to make a small but hopefully 
insignificant semantic change to our public API: option 
VOIKKO_OPT_IGNORE_NUMBERS is now ignored when generating spelling suggestions. 
This change is only significant if an application used this option and tried 
to produce suggestions without checking first whether the word passes spell 
checking. No application that I know of works this way.

There are also new backends available that may be useful for people who are 
developing a specific backend and wish to disable other backends completely 
for easier testing:

Morphology-Backend: null
  This morphology backend returns no analysis for any input.

Speller-Backend: AllOk
  This speller backend returns SPELL_OK for all inputs.

Speller-Backend: AllError
  This speller backend returns SPELL_FAILED for all inputs. This
  could be used when testing a new suggestion backend.

Suggestion-Backend: null
  This suggestion backend returns no suggestions for any input.

Harri