Add tint property for liquids

This allows a customizable tint color for different liquids (e.g. red for lava, blue for water... or wait, is it the other way around? =P).

It can be set by the "tint" property in the block definitions (is that what it's called?).

I'm not sure whether I put the functions in the correct places... And there's probably a lot of style faults (I tried my best though ^^).