Adds directional blood trails

The blood trails now follow the path the body has taken and players can
very easily make out said path.
Heavily damaged corpses will have a much thicker blood trail than
corpses with some brute damage. At the moment, trails overwrite
eachother and a placeholder icon set is used for alien heavy blood
trails.
blood trails now stack

--DO NOT MERGE STILL WIP--
Makes blood trails use overlays, the overlays are contained in a holder
object. DNA is now properly added to them.

todo: redo light, xeno light and xeno heavy trails. help of spriters is
welcome.
Finalizing blood trails

Makes trail_holder a child of cleanables instead of blood (this is to
allow add_blood turf proc to spawn blood splatter on blood trails, also
other kinds of blood effects), also cleaned up the blood code a bit.

Big props to Aranclanos, Cheridan, Pewtershmitz and all other coders on
#coderbus for making this happen.
removes extra unused code
Merged the blood trails from VistaPOWA to the latest tg code