Improve shoot spread math

Several hours of work result in... deciphering basic 3D vector rotation
math!
Add EntityCritical property for arrows

You can now set arrows to 'critical' mode (fancy flying stars and extra
damage)
Also some meta interlinking.

Also, random arbitrary half-add of this to the previous commit... git is
still weird.
Improve Skull_Skin property

Now works for any skull_item rather than only head items.

Also more meta-interlinking