Schema refactoring

[bdunogier]

Schema generation refactoring

Refactors schema generation. Introduces Domain Iterators, proper Schema Workers, a Schema Builder API...

TODO

• [x] specs for the remaining classes, like workers.
• [x] ~spec the AddFieldValueToDomainContent worker more thoroughly (Field Value Builders)~ (postponed for refactoring, see)
• [x] test the the actual schema that gets generated :)
• [x] Update the documentation for extensibility changes
• [x] Update ezsystems/ezplatform-graphql-page (released a v0.1 that requires v0.3)
• [x] ~Update ezsystems/ezplatform-graphql-form~
• [x] Update ezsystems/ezplatform-query-fieldtype (released a v0.1 that requires v0.3)