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'.
- 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`