This allows to get rid of the last remaining Ptr<T>::addref calls.

Also changed the static functions to inline, to stop GCC complaining
about unused functions.

Now the features2d class wrappers use composition instead of inheritance.

This and several following commits contain the "boring" changes required
to support the new Ptr. These are changes like:

* new T -> makePtr<T> or .reset(new T) or Ptr<T>(new T)
  (depending on the situation)
* p.empty() -> !p
* delete_obj -> DefaultDeleter::operator()

and similar changes that are numerous, but primitive.