Completely rewrote the liquid transformation. The algorithm now exclusively modifies the current node in one iteration. Another change is that MapNode.param2 now stores a bit that tells other nodes if a flowing liquid node flows downwards. This is accomplished by two masks on param2 for the "flow down" bit and the "liquid level" bits.

This will be the base of future improvements to the liquid flow. However, in the current state flowing liquid does not always disappear when cut off its source. Be aware that this branch is work in progress.

- When flowing liquid starts to flow down, it stops contributing to its neighboring nodes' liquid level (possibly transforming it back to air)
- Flowing liquid turns into a source if there are at least 2 adjacent liquid source nodes
- If a new liquid flow reaches existing flowing liquid, the existing liquid is now updated properly.

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

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