Builders should not be singletons
Initialize BuildableOrder after creation
Add some checks
Register PageGroupDescription with Json
Register PageDescription with Json
Also register factories in AppModule
Use TypeLiteral instead of TypeToken
No need for ModelFactories!
Register QuestionDescription with Json and AppModule
Register SequenceDescription with Json and AppModule
Register Sequence with Json and AppModule
Add checks and initializations
Json registration

* register Page, PageGroup, and Question with Json and AppModule
* check what is serialized from these
* work around the recursivity of sequence in Page
Thread safety and recursivity of sequence in Question
Fix inject import
Register all answer classes with Json and AppModule