Add third person view

doc/lua_api.txt

minetest.conf.example

src/camera.cpp

src/camera.h

src/client.cpp

src/clientmap.cpp

src/clientserver.h

src/content_cao.cpp

src/defaultsettings.cpp

src/game.cpp

src/game.h

src/hud.cpp

src/localplayer.cpp

src/localplayer.h

src/player.h

src/script/lua_api/l_object.cpp

src/script/lua_api/l_object.h

src/server.cpp

src/server.h