Pretty much...  The code is in place.  Still more work to do.    But it
has a lot of hooks and variables in other code, so commit and merge
before I pick up a million 'Conflicts'.

As it turns out...   an unsigned int can not go to less than 0,  so the
loop never terminates.

- Make all `unified_bed_leveling` data/methods static
- Move some UBL-related variables into the class
- Replace `map_[xy]_index_to_bed_location` with `mesh_index_to_[xy]pos`

UBL's maps are cleaner.   And with a 1 value, G29 O 1  will generate a
map suitable to be Cut & Pasted into Excel so a Suface Map can be
generated.

The wait_for_user change totally broke UBL.   But there is stuff wrong
now in the thermal code and/or LCD Panel code.