- When editing on the LCD, minus now means below the nozzle
- Stores the opposite sign in EEPROM, so bumped to “V20”

This makes `M421` more amenable for irregular matrices

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

- Patched up for most included configurations

- Also patch up some warnings

- Init `zprobe_zoffset`
- Remove `current_position[Z_AXIS] = zprobe_zoffset` lines from the
`set_bed_level_equation_*` functions
- Apply standards to `mesh_bed_leveling` files
- Document `MESH_BED_LEVELING`

Z_DUAL_ENDSTOPS is a feature to enable the use of 2 endstops for both Z
steppers - Let's call them Z stepper and Z2 stepper.
That way the machine is capable to align the bed during home, since both
Z steppers are homed.
There is also an implementation of M666 (software endstops adjustment)
to this feature.
After Z homing, this adjustment is applied to just one of the steppers
in order to align the bed.
One just need to home the Z axis and measure the distance difference
between both Z axis and apply the math: Z adjust = Z - Z2.
If the Z stepper axis is closer to the bed, the measure Z > Z2 (yes, it
is.. think about it) and the Z adjust would be positive.
Play a little bit with small adjustments (0.5mm) and check the
behaviour.
The M119 (endstops report) will start reporting the Z2 Endstop as well.

- Address #1262 by leaving values as set
- Rename `add_homing` to `home_offset`

Tried to make it a little intelligent.

Added option to set Travel Acceleration (non printing moves).
The M204 options was a non sense (S for printing moves and T for retract
moves).

It has been changed to:
P = Printing moves
R = Retract only (no X, Y, Z) moves
T = Travel (non
printing) moves

I will add this info o G-Code wiki in reprap.org. I also advise to put
this info in Marlin next version changelog.

- Also adds a language string for the error.
- Also adds SERIAL_EOL as an alias for SERIAL_ECHOLN(“”)

Should be reading 4 x 4 PID values instead of 4 x 3

Update ConfigurationStore to always write dummy values for disabled
options, including FWRETRACT, DELTA, and SCARA. Update the EEPROM
version to “V15.” Also fixes a buffer overrun with axis_scaling in
Config_ResetDefault.

Moved #endif to allow filament settings to print.

Added PIDTEMP dummy write to align EEPROM read/write operation.

Done

Show Custom M Code when entering M503

* Adds config parameter `PID_PARAMS_PER_EXTRUDER` - allows single PID
parameters to be used where this would be preferable (e.g. dual
identical extruders)
* When disabled, will use `float Kp, Ki, Kd, Kc;` as before.
Preprocessor macros used to switch between.
* ultralcd.cpp defines extra menus for extra parameters only where
required
* M301 reports `e:xx` only if independent pid parameters enabled
* EEPROM structure still leaves space for 3 extruders worth, when undef
will save single parameter to all extruder positions, but only read the
first
* Switching off saves approx 330 B with no LCD enabled, 2634B with LCD
(RRD) enabled: this is significant.
* LCD modifications should be tested.

* R-H reads incorrect M301 line from EEPROM output if more than one is
present. Reverted to original output format, using only E0 value.

* Variables Kp, Ki, Kd, Kc now arrays of size EXTRUDERS
* M301 gains (optional, default=0) E parameter to define which
extruder's settings to modify. Tested, works with Repetier Host's EEPROM
config window, albeit only reads/updates settings for E0.
* All Kp, Ki, Kd, Kc parameters saved in EEPROM (version now v14), up to
3 extruders supported (same as Marlin in general)

Changed AUTO_BED_LEVEL to AUTO_BED_COMPENSATION