This is an update of MarlinDev PR #196.

G20/21: support for switching input units between millimeters and
inches.
M149: support for changing input temperature units.

In support of these changes, code_value() and code_value_short() are
replaced with an array of functions which handle converting to the
proper types and/or units.

Rename some auto/locals to avoid name conflict

Cleanup and naming for number-to-string functions

MBL: cel => cell

[Language] Update Czech 2016/06/07

Use e_steps[EXTRUDERS] instead of e_steps[4]

Alternative impl. of LCD-based manual movement

SINGLENOZZLE: EXTRUDERS versus HOTENDS

Fix a typo in #3955

MBL: Keep active when homing single axes (#3750)

Improved LCD contrast handling

Apply static to remaining Stepper methods

last of missing defines added

Fix M428 width DELTA & SCARA

Galician language update.

Raise the servo probe before stow outside ABL context

need to check line 101-103

set_current_position_from_planner() after stepper.quick_stop()

Apply static to Stepper class

Apply static to Planner class

Apply static to Temperature class

Combine ST7565 / U8GLIB_NHD_C12864 items in dogm

Based on MarlinFirmware/MarlinDev#200 from @eboston