* Updates for ensure_safe_temperature

* M600 fixes

I recently upgraded my Marlin to V1.1.x and decided to publish my fix to make marlin compile properly when #define board megatronics 2 in combination with #define reprapworld keypad v1.1. It was previously described in the comments of a closed pull request. MarlinFirmware#472. All the buttons on the keypad are now operational.

* [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

There was quite a lot to translate this time. I'm not absolutely confident with the whole UBL stuff and it's a PITA to translate. I don't use any kind of levelling myself as i just have a perfectly level cast aluminium plate as bed.
So maybe someone who really uses it might take a closer look at it after it's merged.

Line 7830 prevented successfull compilation when #define CASE_LIGHT_ENABLE is uncommented

...by moving `homing_feedrate_mm_s` to PROGMEM.

Show the Leveling system in use on the About Printer screen.

Ref #5139

* 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

One click option to manually build UBL mesh.

* Save up to 94 bytes of RAM on 20x4 LCD Display machines

Moved the custom screen characters out of RAM into Program Memory.  With
SD-Card support and the Progress Bar enabled, this saves 94 bytes of RAM
memory.

This was tested using the example_configurations/FolgerTech-i3-2020
files.  So a couple small changes to those files snuck into this Pull
Request.

Probably...  We can find similar savings in the Graphics LCD code it we
comb through it.   And if so...  That is the place we really need to
save RAM memory!

* Tidy up white space and indentation