Adding raft surface support

This adds support for rafting surface, with the new following parameters:

* `raftBaseSpeed`, the speed of the very first layer of raft
* `raftSurfaceThickness`, the thickness of each raft surface layers
* `raftSurfaceLinewidth`, the line width of the raft surface
* `raftSurfaceLineSpacing` the line spacing of the raft surface
* `raftSurfaceLayers` the number of raft surface layers
* `raftSurfaceAirGap` the "air gap", i.e the height to skip between surface and the model

My configuration is:
```
raftBaseThickness: 300
raftBaseLinewidth: 1000
raftLineSpacing: 3000
raftBaseSpeed: 8

raftInterfaceThickness: 250
raftInterfaceLinewidth: 350
raftInterfaceLineSpacing: 1000

raftSurfaceThickness: 250
raftSurfaceLineWidth: 400
raftSurfaceLineSpacing: 400
raftSurfaceLayers: 2
raftAirGap: 200
```

(I'm also printing with extrusionWidth 300, speed 40, insetX/0 20 and initialSpeedupLayers 0)

With these settings, we may offer a third raft option in Cura "Raft+surface"

See also issue #61