Add Settings interface for Lua

doc/lua_api.txt

doc/menu_lua_api.txt

src/script/lua_api/CMakeLists.txt

src/script/lua_api/l_settings.cpp

src/script/lua_api/l_settings.h

src/script/lua_api/l_util.cpp

src/script/lua_api/l_util.h

src/script/scripting_game.cpp

src/script/scripting_mainmenu.cpp