Retract filament after print to prevent burned filament

[peteruithoven]

Currently we preheat so that in a demo context users can print rapidly, but this means that the filament in the nozzle gets heated while it's not extruded. This might lead to a clogged nozzle.
Maybe we could retract the filament a bit (putting it out of the heated part) after each print?
I believe Cura / Ultimaker >2 also do this, probably because this enables you to remove the filament without heating the nozzle.

Also see: Disable pre-heating by default