More data in UBL class, make it a static class

- 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`

Marlin/Marlin.h

@@ -430,4 +430,8 @@ void do_blocking_move_to_x(const float &x, const float &fr_mm_s=0.0);
 void do_blocking_move_to_z(const float &z, const float &fr_mm_s=0.0);
 void do_blocking_move_to_xy(const float &x, const float &y, const float &fr_mm_s=0.0);
 
+#if ENABLED(Z_PROBE_ALLEN_KEY) || ENABLED(Z_PROBE_SLED) || HAS_PROBING_PROCEDURE || HOTENDS > 1 || ENABLED(NOZZLE_CLEAN_FEATURE) || ENABLED(NOZZLE_PARK_FEATURE)
+  bool axis_unhomed_error(const bool x, const bool y, const bool z);
+#endif
+
 #endif //MARLIN_H