The gcode reader assumed that each layer starts with a move to the initial position but
for spiralized code that isn't true because the previous layer always ends up in the right
location. So we now start each layer with a fake move to the end position of the previous layer.
This won't actually cause a real move to occur but it ensures that the first line segment
in the new layer has the correct initial point.
CURA-4741
With the multi build plate feature, scene.gcode_list is now a dict which
stores a list of gcode for a build plate, so it makes more sense to have
it renamed to "gcode_dict" because it's not a list.
