Moved some mapnode content stuff from mapnode.{h,cpp} and digging property stuff from material.cpp to content_mapnode.{h,cpp}

Reduced server CPU usage on NodeMetadata step()s. Also furnace now cooks while no players are near it.

--HG--
rename : data/tool_stpick.png => data/tool_stonepick.png
rename : data/tool_wpick.png => data/tool_woodpick.png