several instances need to be created.
- In the ThreeMFWorkspaceReader we need to create some temporal
instances of Preferences that makes it not singleton anymore.
- The current preferences are kept in the Application class and so all
the calls to the preferences are changed to get the preferences from
Application.
- The method getInstance in Preferences is kept as deprecated since some
external plugins.
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.
