lib-fts: Add Unicode TR29 rule WB5a setting to tokenizer. Splits prefixing contracted words from base word. E.g. "l'homme" -> "l" "homme". Together with a language specific stopword list unnecessary contractions can thus be filtered away. This is disabled by default and only works with the TR29 algorithm. Enable by "fts_tokenizer_generic = algorithm=tr29 wb5a=yes"

lib-fts: Add note about possible additional apostrophe.

lib-fts: Moved IS_APOSTROPHE() to fts-common.h