so-5.4.0: New helper method so_make_state of agent_t class

New helper method so_5::rt::agent_t::so_make_state() has been added to so_5::rt::agent_t class. It allows to initialize state_t object immediately after declaration:

class my_agent_t : public so_5::rt::agent_t
   {
   private :
      // Anonymous state.
      const so_5::rt::state_t st_first = so_make_state();
      // Named state.
      const so_5::rt::state_t st_second = so_make_state( "second" );
      ...
   };