C++11 explicit conversion from accessor to bollean

Summary:
Replaced the "safe bool idiom" by explicit conversion to bollean,
in order to follow the improved conventions of the C++11.

Reviewers: felipealmeida, cedric, woohyun, smohanty, raster

Reviewed By: raster

CC: savio, cedric

Differential Revision: https://phab.enlightenment.org/D903

src/bindings/eina_cxx/eina_accessor.hh

@@ -59,10 +59,7 @@ struct accessor
   {
     std::swap(_impl, other._impl);
   }
-private:
-  typedef Eina_Accessor*(accessor<T>::*unspecified_bool_type)() const;
-public:
-  operator unspecified_bool_type() const
+  explicit operator bool() const
   {
     return native_handle() ? &accessor<T>::native_handle : 0 ;
   }