Simplify & clean up API

Things to do:

• Simplify variant setter/getter: Implicitly get all variants instead of the default variant. Remove ability to choose default behavior from getter.
• Settings service: Return NULL on non-existent setting.
• Nomenclature: Use 'sitemap' name for objects and 'variant' name for sitemap ID
• Simplify/clean up CustomLinkManager & EntityManager API method signatures and return values