Add drowning

doc/lua_api.txt

games/minimal/mods/default/textures/bubble.png

src/client.cpp

src/client.h

src/environment.cpp

src/environment.h

src/game.cpp

src/hud.cpp

src/hud.h

src/nodedef.cpp

src/nodedef.h

src/player.cpp

src/player.h

src/script/common/c_content.cpp

src/script/lua_api/l_object.cpp