CObj: Using abstract classes annotated with CObj doesn't work as expected

Mixing abstract CObj classes in a hierarchy with concrete CObj classes exhibits strange behavior:
```scala
@CObj
class GObject
@CObj
abstract GtkWidget extends GObject
@CObj
class GtkWindow extends GtkWidget
```
aince in this case for `GObject` the empty pparams constructor is called during instantiation of `GtkWindow`, when in fact `GObject(__ref: CRef[Byte])` would need to be called.