FWRETRACT in mm/s

Firmware retraction now stores the retract and recover speeds in mm/s
instead of mm/min. This makes it match the units of the maximum
feedrate, and fixes problems with modifying the value via LCD control
panel. From gcode, the values are still taken in mm/min to match the
units of G1 and similar, and they are converted to mm/s before they are
stored.

I also lowered the default retract feedrate to make it less likely to
cause problems for geared extruders when the user hasn’t bothered to
set a reasonable maximum feedrate, though users should be setting both
of these values to suit their hardware.

Marlin/Configuration_adv.h

@@ -402,10 +402,10 @@ const unsigned int dropsegments=5; //everything with less than this number of st
 #ifdef FWRETRACT
   #define MIN_RETRACT 0.1                //minimum extruded mm to accept a automatic gcode retraction attempt
   #define RETRACT_LENGTH 3               //default retract length (positive mm)
-  #define RETRACT_FEEDRATE 80*60         //default feedrate for retracting
+  #define RETRACT_FEEDRATE 45            //default feedrate for retracting (mm/s)
   #define RETRACT_ZLIFT 0                //default retract Z-lift
   #define RETRACT_RECOVER_LENGTH 0       //default additional recover length (mm, added to retract length when recovering)
-  #define RETRACT_RECOVER_FEEDRATE 8*60  //default feedrate for recovering from retraction
+  #define RETRACT_RECOVER_FEEDRATE 8     //default feedrate for recovering from retraction (mm/s)
 #endif
 
 //adds support for experimental filament exchange support M600; requires display