Refactore for new DCG API.
Add dependency symfony/property-access.
Refactore, make compatible with DCG 2

Refactore the Entity base class, use an interface instead now.
Make compatible with the new DC_General 2 API.
Guess entity property type and convert sub-entities if PK is given.
Use EntityInterface instead of class constants.
Ignore proxy classes, when fetch all properties.
Raise the dev-develop version to 2.0.x-dev.
Remove unused parameter.
Add support for indexes and uniques.
Prevent exceptions in backend when the entity classes are not up to date.
Add EntityAccessor::getPrimaryKeyValues() method.
Fix pass wrong value when generating primary key.
Add EntityAccessor::getPublicProperty() method.
Add EntityAccessor::getPublicProperties() method.
Reformat code.
Replace symfony serializer with jms serializer.
Add method EntityHelper::getEntityAccessor().
Add helpers to load/save entity options.
Fix the jms/serializer version constraint.
Replace the symfony serializer with jms serializer.