ImapDb.Database: Register new ICU-based tokeniser for FTS

The SQLite tokeniser does not deal with scripts that do not use spaces
for word breaking (CJK, Thai, etc), thus searching in those languages
does not work well.

This adds a custom SQLite tokeniser based on ICU that breaks words for
all languages supported by that library, and uses NFKC_Casefold
normalisation to handle normalisation, case folding, and dropping of
ignorable characters.

Fixes #121

sql/version-030.sql

@@ -14,6 +14,6 @@ CREATE VIRTUAL TABLE MessageSearchTable USING fts5(
     bcc,
     flags,
 
-    tokenize="unicode61 remove_diacritics 2",
+    tokenize="geary_tokeniser",
     prefix="2,4,6,8,10"
 )