Added support to disable built-in HUD elements

doc/lua_api.txt

src/client.cpp

src/client.h

src/clientserver.h

src/game.cpp

src/hud.cpp

src/hud.h

src/player.cpp

src/player.h

src/scriptapi_object.cpp

src/scriptapi_object.h

src/server.cpp

src/server.h