Fix & improve FTS. Implements #552 (#556)

Improve free text search by:
 * Fixing n-gram generation when a space is added in front of a word
 * give 4-grams a higher weight than multiple matches of the same 3-gram by not counting duplicate n-grams (as a result the score can not be >1 anymore)
 * lower the min-score to 0.33 which means at least 1 out of 3 n-grams must match in order for a task to be considered a result

The changes are supposed to favor longer matches over many shorter matches.