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/Configuration_adv.h

@@ -270,6 +270,17 @@
 // Enable the option to stop SD printing when hitting and endstops, needs to be enabled from the LCD menu when this option is enabled.
 //#define ABORT_ON_ENDSTOP_HIT_FEATURE_ENABLED
 
+// Babystepping enables the user to control the axis in tiny amounts, independently from the normal printing process
+// it can e.g. be used to change z-positions in the print startup phase in realtime
+// does not respect endstops!
+
+//#define BABYSTEPPING
+//#define BABYSTEP_XY  //not only z, but also XY
+
+#ifdef COREXY
+    #error BABYSTEPPING not implemented for COREXY yet.
+#endif
+
 // extruder advance constant (s2/mm3)
 //
 // advance (steps) = STEPS_PER_CUBIC_MM_E * EXTUDER_ADVANCE_K * cubic mm per second ^ 2