Configure Firmware Retraction

Firmware retraction can now be enabled via the Retraction LCD menu.  It
can also be enabled by issuing a M209 S1 command.  To disable issue M209
S0.

I've corrected the LCD display issue and the feed rates are now in
mm/sec with a maximum value of 100 mm/sec.

For this feature to work you slicer must have retraction enabled and
some value for the amount of retraction.  When the firmware sees a
retraction event from the slicer the settings will be replaced with
those in the firmware.

On the fly changes to retraction can be made via the LCD.  The can also
be changed with gcode as below:

M207 - set retract length S[positive mm] F[feedrate mm/sec] Z[additional
zlift/hop]
M208 - set retract recover length S[positive mm surplus to the M207 S*]
F[feedrate mm/sec]