Proposal for slingers

[Trve Leveller]

@Vartan: I did not implied trajectory and accuracy are linked in the engine. I proposed to change trajectory and accuracy, as two different things to do.
And accuracy can be changed easily, I already did this for my personal modification (added 15.-17. century units) and it works. And IIRC trajectory is linked to speed in M2TW. (Have to confirm that later, I dont have m2tw on my pc right now, my external hard drive is dead).