Implementation of FW extruder change retract

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.
Merge pull request #948 from Dim3nsioneer/Marlin_v1

Implementation of FW extruder change retract