Implementation of FW extruder change retract

This is a possible solution for the FW extruder change/swap retraction.
FW retraction is extended onto swap retraction invoked by 'G10 S1'.
Bookkeeping of the retract state of all extruders allows for having one
extruder fw standard retracted while another extruder is swap retracted.
An LCD menu item for the swap retract and recover length was added.
It has been tested on an Ultimaker Original with Dual Extruder.