Add a generic iteration method to the immutable List abstraction in JavaHL,
and allow for lists containing more subtypes of jobject.

[in subversion/bindings/javahl/native]
* jniwrapper/jni_list.hpp
  (BaseList::m_contents): Make protected so that the template can use it.
  (List, MutableList): Add a NativeT template parameter that defaults to
   jobject, and use it to convert the stored type in indexing operators.
  (List::for_each): New template method; uses std::for_each.
  (List::FunctorAdapter): Adapter for the for_each functor parameter.

* org_apache_subversion_javahl_util_PropLib.cpp
  (UnparseFunctor): New; functor class for unparsing externals.
  (Java_org_apache_subversion_javahl_util_PropLib_unparseExternals):
   Use List::for_each with UnparseFunctor instead of explicitly
   iterating over the list.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1542219 13f79535-47bb-0310-9956-ffa450edef68