As well a couple of minor changes to handling of extruder offset for
dual x-carriage mode.

Also added a couple of missed merge lines in homeaxis() from previous
commit.

add more delta defaults
on delta make second home even slower

Dual x-carriage designs offer some substantial improvements for dual
extruder printing.

Untested

M109 S180 waits only when heating.
M109 R180 also waits when cooling.

Diffed and merged, preserving my updates

Integrated fixes from henrikbrixandersen pull request #504

Reviert previous change of #if BLAH_PIN > 0 to #if defined(BLAH_PIN) &&
BLAH_PIN > -1. Unfortunately some times pin 0 is used. For my sins I've
gone through and replaced all unsafe checks of #if BLAH_PIN > -1 with
the safe version.

The for loop for Axis count over the Array.
Add a break after M907

Extruder 2 was checked where extruder 1 should be checked.

If S is left out perhaps it should be silent, but check the spec for
this M code.

Also refactor extruder auto fan logic to simplify further and now only
check every 2.5 seconds

Argument order of lcd_buzz was set to mirror that on tone(). Change
default M300 frequency to something audible.

Add new 'callback' edit-menu types that call a function after the edit is done. Use this to display and edit Ki and Kd correctly (removing the scaling first and reapplying it after). Also use it to reset maximum stepwise acceleration rates, after updating mm/s^2 rates via menus. (Previously, changes did nothing to affect planner unless saved back to EEPROM, and the machine reset).

Add calls to updatePID() so that PID loop uses updated values whether set by gcode (it already did this), or by restoring defaults, or loading from EEPROM (it didn't do those last two). Similarly, update the maximum step/s^2 accel rates when the mm/s^2 values are changed - whether by menu edits, restore defaults, or EEPROM read.

Refactor the acceleration rate update logic, and the PID scaling logic, into new functions that can be called from wherever, including the callbacks.

Add menu items to allow the z jerk and e jerk to be viewed/edited in the Control->Motion menu, as per xy jerk.

Conflicts:
	Marlin/language.h

The LARGE_FLASH flag allows superfluous features to be excluded
on boards with less memory

Disable the high current output and wait a little before to turn off,
because the interrupting capacity of the PSU is unknown.
Could be a function if needed by other.

From the prepare menu, accessible when is not printing, you have the
possibility to turn off the PSU when is on et vice versa.
From the host, you can turn off or turn on the PSU then the menu is
updated accordingly.
From the LCD message, the printer status is reported ready or off
respectively when the PSU is on or off.

This change allows fan outputs to automatically turn on/off when the
associated nozzle temperature of an extruder is above/below a threshold
temperature.
Multiple extruders can be assigned to the same pin in which case the fan
will turn on when any selected extruder is above the threshold.
It also makes the M42 command compatible with the M106/M107 command.
The majority of the logic in this change will be evaluated by the
compiler at build time (i.e, low code space requirements).

This includes refactoring the buttons code to remove a large amount of
non-pin dependent duplication from pins.h.

The extruder offset can be specified in the configuration
file or adjusted on the fly using the "M218 T# X# Y#" command.
The EEPROM support is not yet merged in.
The "T#" command can take option "F#" that specifies the feedrate
at which the printing head should be re-positioned. If not
specified the re-positioning move is not preformed immediately,
but the coordinates are adjusted for the printer to properly
position the head when the next movement happens.

This reverts commit 01541ee64c.

[default off for now]
syntax: M600  X[pos] Y[pos] Z[relative lift] E[initial retract] L[later retract distance for removal]

if enabled, after a M600, the printer will retract by E, lift by Z, move to XY, retract even more filament.
Oh, and it will display "remove filament" and beep like crazy.

You are then supposed to insert a new filament (other color, e.g.) and click the display to continue.
After having the nozzle cleaned manually, aided by the disabled e-steppers.
After clicking, the printer will then go back the whole shebang, and continue printing with a fancy new color.