Allow Particlespeed/Particlespeedspread to be negative

[Karl]

Currently they allow only be positive
when they get a negative value they disappear and doesn't get rendered at all.

Eventually delay operators would be good too (T) maybe
so for example 5 T5 -5
so it would have it's value for 5 which last for 5 frames then it's value gets set to -5, would maybe be possible to simulate a tornado or fire whirl.