Changes for graphics displays

Replaced displaying "---" instead of the value of a coordinate when
unhomed or with reduced precision
with blinking the coordinate-prefix-character ('X','Y','Z').
For "unhomed" a '?' is shown every second second - until that axis is
homed. The value displayed is, as before the "---" where displayed, the
relative to the reset position coordinate value.
When the axis stepper was disabled, now we can display a hint on that,
by showing a blinking ' ' instead of the axis letter, when
WARN_REDUCED_ACCURACY is defined.

I suppose the code itself is here the better documentation.

A '+/-' character is in non of our charsets so i decided for a '?' for
now to reduce the work.
There is no additional space on the displays one could use to display
the information, so replacing something is the only option. As the axis
letters are totally redundant with their positions on the display they
contain the least information.
So my decision was to overwrite them.

Marlin/Configuration.h

 #define Z_ENABLE_ON 0
 #define E_ENABLE_ON 0 // For all extruders
 
-// Disables axis stepper immediately  when it's not being used.
+// Disables axis stepper immediately when it's not being used.
 // WARNING: When motors turn off there is a chance of losing position accuracy!
 #define DISABLE_X false
 #define DISABLE_Y false
 #define DISABLE_Z false
+// Warn on display about possibly reduced accuracy
+//#define WARN_REDUCED_ACCURACY
 
 // @section extruder
 

Marlin/dogm_lcd_implementation.h

     }
 
   // X, Y, Z-Coordinates
+  // Before homing the axis letters are blinking 'X' <-> '?'.
+  // When axis is homed but axis_known_position is false the axis letters are blinking 'X' <-> ' '.
+  // When everything is ok you see a constant 'X'.
   #define XYZ_BASELINE 38
   lcd_setFont(FONT_STATUSMENU);
 
   #endif
   u8g.setColorIndex(0); // white on black
   u8g.setPrintPos(2, XYZ_BASELINE);
-  lcd_print('X');
+  if (blink & 1)
+    lcd_printPGM(PSTR("X"));
+  else {
+    if (!axis_homed[X_AXIS])
+      lcd_printPGM(PSTR("?"));
+    else
+      #if ENABLED(WARN_REDUCED_ACCURACY)
+        if (!axis_known_position[X_AXIS])
+          lcd_printPGM(PSTR(" "));
+        else
+      #endif
+      lcd_printPGM(PSTR("X"));
+  }
   u8g.drawPixel(8, XYZ_BASELINE - 5);
   u8g.drawPixel(8, XYZ_BASELINE - 3);
   u8g.setPrintPos(10, XYZ_BASELINE);
-  if (axis_homed[X_AXIS] || (blink & 1))
-    lcd_print(ftostr31ns(current_position[X_AXIS]));
-  else
-    lcd_printPGM(PSTR("---"));
+  lcd_print(ftostr31ns(current_position[X_AXIS]));
+
   u8g.setPrintPos(43, XYZ_BASELINE);
-  lcd_print('Y');
+  if (blink & 1)
+    lcd_printPGM(PSTR("Y"));
+  else {
+    if (!axis_homed[Y_AXIS])
+      lcd_printPGM(PSTR("?"));
+    else
+      #if ENABLED(WARN_REDUCED_ACCURACY)
+        if (!axis_known_position[Y_AXIS])
+          lcd_printPGM(PSTR(" "));
+        else
+      #endif
+      lcd_printPGM(PSTR("Y"));
+  }
   u8g.drawPixel(49, XYZ_BASELINE - 5);
   u8g.drawPixel(49, XYZ_BASELINE - 3);
   u8g.setPrintPos(51, XYZ_BASELINE);
-  if (axis_homed[Y_AXIS] || (blink & 1))
-    lcd_print(ftostr31ns(current_position[Y_AXIS]));
-  else
-    lcd_printPGM(PSTR("---"));
+  lcd_print(ftostr31ns(current_position[Y_AXIS]));
+
   u8g.setPrintPos(83, XYZ_BASELINE);
-  lcd_print('Z');
+  if (blink & 1)
+    lcd_printPGM(PSTR("Z"));
+  else {
+    if (!axis_homed[Z_AXIS])
+      lcd_printPGM(PSTR("?"));
+    else
+      #if ENABLED(WARN_REDUCED_ACCURACY)
+        if (!axis_known_position[Z_AXIS])
+          lcd_printPGM(PSTR(" "));
+        else
+      #endif
+      lcd_printPGM(PSTR("Z"));
+  }
   u8g.drawPixel(89, XYZ_BASELINE - 5);
   u8g.drawPixel(89, XYZ_BASELINE - 3);
   u8g.setPrintPos(91, XYZ_BASELINE);
-  if (axis_homed[Z_AXIS] || (blink & 1))
-    lcd_print(ftostr32sp(current_position[Z_AXIS]));
-  else
-    lcd_printPGM(PSTR("---.--"));
+  lcd_print(ftostr32sp(current_position[Z_AXIS]));
   u8g.setColorIndex(1); // black on white
 
   // Feedrate