Key Area now use Requirement, instead of QuestProgress

* Requirement class moved from model.conversation to model.script
* Added requirement questLatestProgress : quest stage must be reached, but no stage with superior ID must be reached.
* New key requirements conditions type (item/gold/bonemeals) consumption (more or less than value, never strict checking, either <= or >=).
Merge branch 'key_area_use_requirements'
Add explicit quest stages to existing key areas
Egocentric commit. Added my name in the authors...