Allow control of cut direction (in to out, out to in) when clearing voids while roughing

[foxharp]

As per discussion on forum:

It's really only the final layer of clearing where it matters.

And, as an aside, i couldn't see(version 4.1.5) that the "conventional" checkbox made any difference (while roughing) to my voids -- the layers were cut in alternate directions. (There was no finishing pass configured.)

[foxharp]

Possibly related to #275

[MROLLS1]

This issue is directly related to one of the ones I was going to add. I would like to be able to control the toolpath generation a bit more, starting with this. As a basic example, if I want to take 1mm off the top face of the stock and I pick a LEVEL operation, I get a toolpath that looks like a digital wavesign - tool starts at origin, plunges down, traverses in Y+ and then stops, steps over in X, and traverses back in Y-. This means that the cutter, spinning clockwise, is climb milling as the spindle travels in Y+ and conventional milling when it travels in Y-.

To prevent this, we would want to either traverse in Y+, raise Z and jog back to the start point, step over, and come down and traverse in Y+ again and effectively stripe our way across the part..... Or, more efficiently, we would spiral in from the origin/start point to the centre, then plunge and spiral back out (if using an end mill capable of plunging) or jog to the startpoint, plunge and spiral from there to the centre again if using a tool not capable of plunging (ie, a flycutter, or router surfacing bit)