message: attribute: Apply accessibility to signal attribute too.
diff: #define ELLE_ATTRIBUTE_PROPERTY_public_SIGNAL_DECLARE(Type, Name)
  #define ELLE_ATTRIBUTE_PROPERTY_static_SIGNAL_DECLARE(Type, Name)
  #define ELLE_ATTRIBUTE_PROPERTY_signal_SIGNAL_DECLARE(Type, Name) \
  + boost::signals2::signal<void (ELLE_ATTRIBUTE_STRIP_PARENS(Type))> \
  + BOOST_PP_CAT(_, BOOST_PP_CAT(Name, _changed)); \
  public: \
  auto \
  BOOST_PP_CAT(Name, _changed)( \
  { \
  return \
  this->BOOST_PP_CAT(_, BOOST_PP_CAT(Name, _changed)).connect(cb); \
  - } \
  - private: \
  - boost::signals2::signal<void (ELLE_ATTRIBUTE_STRIP_PARENS(Type))> \
  - BOOST_PP_CAT(_, BOOST_PP_CAT(Name, _changed));
  + }
  #define ELLE_ATTRIBUTE_PROPERTY_virtual_SIGNAL_DECLARE(Type, Name)
  #define ELLE_ATTRIBUTE_PROPERTY__SIGNAL_HAS
  