Add event manager and use it to trigger sounds

games/mesetint/mods/default/init.lua

src/camera.cpp

src/client.cpp

src/client.h

src/event.h

src/event_manager.h

src/game.cpp

src/gamedef.h

src/nodedef.cpp

src/nodedef.h

src/player.cpp

src/scriptapi.cpp

src/server.cpp

src/server.h

src/sound.h