The long descriptor will still be available in M115

Apply `ENABLED` / `DISABLED` macros to files needing only a small
number of changes.

- Use `setTargetedHotend` in `M200`, as with other commands that use
`T` for the extruder
- Synthesize the "invalid extruder" message, obviating several long
strings

This reverts the changes done to the error message for line number mismatches done in commit d43cc2dd5f

- Also move audio feedback into the command
- Added shorthand for `lcd_buzz` availability

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

- Add `M111` as a standard option to set the debug level
- Implement `DEBUG_ECHO` in `process_commands`
- Other debug levels (e.g., `DEBUG_DRYRUN`) need more work

- Replace “Mendel” with “RepRap”

- Patched up for most included configurations

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.