Include of language_en.h now behind national language.

All "#define MSG_STR_*" now surrounded by #ifndef and #endif.

+ May decrease the workload for feature developers. They may omit to
update all the language files but "language_en.h". When loading _en
first, all new features give some display, but the new in English.

- May throw some additional warnings about redefinition during compile.

Select DOGM-displays automaticly.
Give hint where to find charsets.
Add '²' and '³' characters.

This reverts commit 4d5b0e2037.

Defined the charsets plus a default.

- Also partial cleanup of Marlin_main.cpp with a smidgen of Doxygen.

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

## Add: Board for Vellemann K8200 (derived from 3Drag)

* Boards.h: updated comment for board 3Drag - K8200 removed
* Boards.h: added `BOARD_K8200` with ID 78

* language.h.: machine name and firmware URL for new board

* pins.h: added `K8200` to any `#if` with `3Drag` and corresponding
comments

No matter what LANGUAGE_CHOICE was set to it would always take the en branch.
I revamped it a bit to use a macro to create the language include file name. This required a slight tweak to the RU declaration since it's special-cased in a couple places in code.

This is an attempt to pull my changes from #1222 over.

Tested for SAV MKI and SAV 3D LCD on lewihe.

Added boards.h which contains defines for all the board numbers, plus
some shorthand macros for less typing.

Added boards.h which contains defines for all the board numbers, plus
some shorthand macros for less typing.

Breaking up languages.h makes it easier to translate. Using language
codes makes it easier to select a language and sets the naming standard.

Messages in gcode files like “M117 Printing…” were not being cleared on
Stop Print.

Messages in gcode files like “M117 Printing…” were not being cleared on
Stop Print.

Messages in gcode files like “M117 Printing…” were not being cleared on
Stop Print.

Add an item to the LCD menu that applies the current axis position to
the “add_homeing” offset.

The serial protocol has to stay machine readable, without having
to cope with a ton of different human language variants. So
just leave it at the original english version.

Should fix ErikZalm/Marlin#1052

FW retraction is extended onto swap retraction invoked by 'G10 S1'.
Bookkeeping of the retract state of all extruders allows for having one
extruder fw standard retracted while another extruder is swap retracted.
An LCD menu item for the swap retract and recover length was added.

Added:
* new language (Basque-Euskera)
* some minor corrections for Spanish
* added 2 new constants for every language (required for a minor change
I am preparing to ultralcd.cpp)

Updated to latest version.

Added catalan language

Preheat LCD menus base on code by D-Base

Extension of M221, Tune menu

Firmware retraction now stores the retract and recover speeds in mm/s
instead of mm/min. This makes it match the units of the maximum
feedrate, and fixes problems with modifying the value via LCD control
panel. From gcode, the values are still taken in mm/min to match the
units of G1 and similar, and they are converted to mm/s before they are
stored.

I also lowered the default retract feedrate to make it less likely to
cause problems for geared extruders when the user hasn’t bothered to
set a reasonable maximum feedrate, though users should be setting both
of these values to suit their hardware.

- Fixed Few finnish transalations
- Removed duplicate contrast define