* SERIAL_XON_XOFF not supported on USB-native AVR devices

User could enable SERIAL_XON_XOFF on USB-native devices and it would not be enabled without warning, but M115 would report the capability as available.

Example Configuration.h files are not updated yet.   You need to cross
your settings over to the default Configuration.h file in the \Marlin
directory.   (UBL_G26_MESH_VALIDATION enablement has moved to a new
location in the file.)

Based on #7975 and #7979

Changes to support NEOPIXEL LED strips
- Support for different NEOPIXEL as defined in Adafruit_NeoPixel.h
- ability to setup startup brightness
- ability to define sequential/non-sequential transition of color change during heating-up
- additional parameter to M150 P<brightness>

- Original CR-10
- MKS Mini Display

Sanity Check for LCD_FOR_MELZI  would fail with "Please select no more than one LCD controller option" any time #define LCD_FOR_MELZI was enabled in Configuration.h, even if there were no other LCD options enabled.

Replaced extruder with hotend

As proposed in #7429

LCD_BED_LEVELING requires PROBE_MANUALLY with auto bed leveling enabled.  See #7396

Connected as described at http://printrbot.com/shop/led-strip/

Based on patch by Kelly Anderson <kelly@xilka.com> at
http://www.xilka.com/printrbot/marlin/1.1.4/20170707/

DISABLE_[XYZ] is incompatible with HOME_AFTER_DEACTIVATE and Z_SAFE_HOMING

Add support for the PCA9632 PWM color LED driver used on the Ultimaker 2 and Wanhao Duplicator 6.

* Remove ADVANCED_PAUSE_FEATURE requirement for PROBING_HEATERS_OFF

* Add HEATER_IDLE_HANDLER conditional

* [New Feature] I2C position encoder support

I plan to continue improving/cleaning this up, as there areas that need work.

* let the cleanups begin.

* progress

* more progress

* comments, rename files, etc.

* clean

* Cleanups per thinkyhead

* a few more cleanups

* cleanups, bugfixes, etc.

* remove unnecessary passes_test(), additional cleanups/optimizations

* cleanups

* misc.

* Fix up I2CPEM.init() and a few other things.

* organize, fix, rename, etc.

* more optimization

* a few more tweaks

* Remove requirement for LCD when UBL is used.

* fix previous oversights

* further refinement - error messages for P2 & P4

* require R on G26 when not using LCD; default to all points