'First and Last layer' option upgrade

[Yury-MonZon]

Sometimes it's better to turn off combing not only on the first ant last layers, but on a few layers, which could be calculated by 'Top/Bottom Layer Thickness'/'Layer Height'. In my case it is 0.9mm/0.3mm=3 layers from the top and bottom. This provides much smoother surface.

It could be done by changing line 136 from if layer == 0 or layer == NoOfLayers-1: to if (layer < FirstLastLayers): # or (layer >= (NoOfLayers-FirstLastLayers)):

And define FirstLastLayers as a parameter in plugin interface.

I've made it work with this: if layer <= 3 or layer >= (NoOfLayers-3):

But I've couldn't manage to make plugin read and calculate this value by reading 'Top/Bottom Layer Thickness' and 'Layer Height' from Cura settings.