Encapsulate Stepper, Planner, Endstops in singleton classes

Don't reject 8.3-filenames with chars > 0x7f

Prevent stuck M109/M190 when target is changed

Don't mangel 8.3-filenames with chars > 0x7f

Windows produces 8.3filenames wit chars > 0x7f. Those have been rejected
by Marlin until now.

With these 'malformed' filenames can now be worked with:
In the LCD menue
With RepetierHost (V1.6.1 tested) - full support. Characters are
displayed as '?'
With Octoprint (1.2.10 tested) the files do not appear in the files
area. At the console, listed with M20 they appear with a '�'.
With Pronterface the files appear in the sd-window but you can't start
them. They are mangled by pronterface. The names are altered and than
recected by Marlin. In the console they apper with differen but not the
correct characters.

All in all a little step forward.

Fix for #3593

Fix an acceleration anomaly by making locals signed

Print job statistics

Keepalive not needed while waiting for temperatures

Update distribution date and readme again

Correct the REVERSE_MENU_DIRECTION comment

Simplified LCD Language settings

・Change "RC6" to "RCBugFix" on top part of the readme
・Unify distribution date of RCBugFix and RC6
・Move comments about IDE version from RCBugFix section to RC6 section

Correct an error in lang_de

Update distribution date and readme

There is no way to translate "A-travel" to "A Rückzug". Thats simply
wrong.

It sync with the RC.

The fix is simple. Most changes are because of changed indendation,
bacause of leaving early for wrong tool number

We don't have to  avoid German "Umlaute" any more.

General cleanup around high level move functions

- Use new `DEBUG_POS` macro for `DELTA` debug
- Neaten up `prepare_move` a smidgen
- Remove an old commented `prepare_move()` line

Report current position to host after M206 / M428