Add the socalled "Babystepping" feature.

It is a realtime control over the head position via the LCD menu system that works _while_ printing.
Using it, one can e.g. tune the z-position in realtime, while printing the first layer.
Also, lost steps can be manually added/removed, but thats not the prime feature.
Stuff is placed into the Tune->Babystep *

It is not possible to have realtime control via gcode sending due to the buffering, so I did not include a gcode yet. However, it could be added, but it movements will not be realtime then.

Historically, a very similar thing was implemented for the "Kaamermaker" project, while Joris was babysitting his offspring, hence the name.

say goodby to fuddling around with the z-axis.

Marlin/temperature.cpp

@@ -66,6 +66,10 @@ float current_temperature_bed = 0.0;
   unsigned char fanSpeedSoftPwm;
 #endif
   
+#ifdef BABYSTEPPING
+  volatile int babystepsTodo[3]={0,0,0};
+#endif
+  
 //===========================================================================
 //=============================private variables============================
 //===========================================================================
@@ -1252,7 +1256,26 @@ ISR(TIMER0_COMPB_vect)
        bed_max_temp_error();
     }
 #endif
-  }  
+  }
+  
+#ifdef BABYSTEPPING
+  for(uint8_t axis=0;axis<3;axis++)
+  {
+    int curTodo=babystepsTodo[axis]; //get rid of volatile for performance
+   
+    if(curTodo>0)
+    {
+      babystep(axis,/*fwd*/true);
+      babystepsTodo[axis]--; //less to do next time
+    }
+    else
+    if(curTodo<0)
+    {
+      babystep(axis,/*fwd*/false);
+      babystepsTodo[axis]++; //less to do next time
+    }
+  }
+#endif //BABYSTEPPING
 }
 
 #ifdef PIDTEMP