Allow negative retract values in the LCD

This can already be done via G-Code, so adding the capability to the LCD should be straight forward.

Marlin/ultralcd.cpp

@@ -2162,7 +2162,7 @@ KeepDrawing:
       #endif
       MENU_ITEM_EDIT(float3, MSG_CONTROL_RETRACTF, &retract_feedrate_mm_s, 1, 999);
       MENU_ITEM_EDIT(float52, MSG_CONTROL_RETRACT_ZLIFT, &retract_zlift, 0, 999);
-      MENU_ITEM_EDIT(float52, MSG_CONTROL_RETRACT_RECOVER, &retract_recover_length, 0, 100);
+      MENU_ITEM_EDIT(float52, MSG_CONTROL_RETRACT_RECOVER, &retract_recover_length, -100, 100);
       #if EXTRUDERS > 1
         MENU_ITEM_EDIT(float52, MSG_CONTROL_RETRACT_RECOVER_SWAP, &retract_recover_length_swap, 0, 100);
       #endif