Better Naming

Continuing improvements around the naems of concepts in Moo:
- renamed 'getOriginExpression()' to 'getSourcePropertyExpression()' to match the updated usage.
- renamed 'translation' to 'source' on @CollectionProperty to match @Property
- renamed 'itemTranslation' to 'itemClass' on @CollectionProperty. It's more accurate and makes way for item source property expressions.

Related to #82.