Make EEPROM reproducible in GCode

With these changes the output of `M503 S0` is all you need to restore
the EEPROM. Building on this it is straightforward to save and restore
the EEPROM state using the SD card or external GCode file.

- Added `M145` to set “heatup states” for the LCD menu
- Added `M420` to toggle Mesh Bed Leveling
- Added `M421` to set a single Mesh coordinate
- Extended `Config_PrintSettings` with added M codes
- Cleaned up some comments here and there

Marlin/language.h

@@ -158,6 +158,9 @@
 #define MSG_Z_MAX                           "z_max: "
 #define MSG_Z2_MAX                          "z2_max: "
 #define MSG_Z_PROBE                         "z_probe: "
+#define MSG_ERR_MATERIAL_INDEX              "M145 S<index> out of range (0-1)"
+#define MSG_ERR_XYZ_REQUIRED_FOR_M421       "M421 requires XYZ parameters"
+#define MSG_ERR_MESH_INDEX_OOB              "Mesh XY index is out of bounds"
 #define MSG_M119_REPORT                     "Reporting endstop status"
 #define MSG_ENDSTOP_HIT                     "TRIGGERED"
 #define MSG_ENDSTOP_OPEN                    "open"