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.

Consider this a part of merging #1357.