Using header with forward declarations, containing declarations for all
classes with default parameters. The classes themselves don't have the
defaults.
This also allows users to more conveniently forward-declare instead of
digging in sources and writing the declarations on their own.
The downside of this is that the functions are now virtual, which makes
them slower than before. Maybe final qualifier would help?
AbstractObject::sceneObject() is marked as private in Object to avoid
confusion.
Functionality present in Object is now split into three main components:
* Object itself, handling parent/children relationships
* Transformation implementation and interfaces for common functionality
* Object features, providing transformation caching and base for
cameras, collision shapes, rigid bodies etc.
Some functionality depending on former implementation is temporarily
disabled and will be reworked later.