Localize babystepping in the Temperature class

So that `ultralcd.cpp` doesn’t need to worry about the details.

Marlin/temperature.h

@@ -303,6 +303,37 @@ class Temperature {
       #endif
     }
 
+    #if ENABLED(BABYSTEPPING)
+
+      FORCE_INLINE void babystep_axis(AxisEnum axis, int distance) {
+        #if ENABLED(COREXY) || ENABLED(COREXZ)
+          #if ENABLED(BABYSTEP_XY)
+            switch (axis) {
+              case X_AXIS: // X on CoreXY and CoreXZ
+                babystepsTodo[A_AXIS] += distance * 2;
+                babystepsTodo[CORE_AXIS_2] += distance * 2;
+                break;
+              case CORE_AXIS_2: // Y on CoreXY, Z on CoreXZ
+                babystepsTodo[A_AXIS] += distance * 2;
+                babystepsTodo[CORE_AXIS_2] -= distance * 2;
+                break;
+              case CORE_AXIS_3: // Z on CoreXY, Y on CoreXZ
+                babystepsTodo[CORE_AXIS_3] += distance;
+                break;
+            }
+          #elif ENABLED(COREXZ)
+            babystepsTodo[A_AXIS] += distance * 2;
+            babystepsTodo[C_AXIS] -= distance * 2;
+          #else
+            babystepsTodo[Z_AXIS] += distance;
+          #endif
+        #else
+          babystepsTodo[axis] += distance;
+        #endif
+      }
+
+    #endif // BABYSTEPPING
+
   private:
 
     void set_current_temp_raw();