Play_controller: register as AI observer in reset_gamestate()

Now when play controller can outlive AI manager, it needs to register
itself as an observer when the AI manager is recreated.

While I was working with this code, I also moved the existing
registration from playsingle_controller to play_controller (for
consistency) and stopped using the singleton pointer to access the
manager.

Thanks to @gfgtdf for spotting the problem.

src/play_controller.cpp

@@ -216,6 +216,7 @@ void play_controller::init(const config& level)
 		resources::game_events = gamestate().events_manager_.get();
 		resources::lua_kernel = gamestate().lua_kernel_.get();
 
+		gamestate_->ai_manager_.add_observer(this);
 		gamestate_->init(level, *this);
 		resources::tunnels = gamestate().pathfind_manager_.get();
 
@@ -314,6 +315,7 @@ void play_controller::reset_gamestate(const config& level, int replay_pos)
 	resources::game_events = gamestate().events_manager_.get();
 	resources::lua_kernel = gamestate().lua_kernel_.get();
 
+	gamestate_->ai_manager_.add_observer(this);
 	gamestate_->init(level, *this);
 	gamestate().set_game_display(gui_.get());
 	resources::tunnels = gamestate().pathfind_manager_.get();

src/playsingle_controller.cpp

@@ -84,8 +84,6 @@ playsingle_controller::playsingle_controller(const config& level,
 	// game may need to start in linger mode
 	linger_ = this->is_regular_game_end();
 
-	ai::manager::get_singleton().add_observer(this);
-
 	plugins_context_->set_accessor_string("level_result", std::bind(&playsingle_controller::describe_result, this));
 	plugins_context_->set_accessor_int("turn", std::bind(&play_controller::turn, this));
 }