Add "Level Corners" menu option

Marlin/Conditionals_post.h

Marlin/Configuration.h

.../example_configurations/AlephObjects/TAZ4/Configuration.h

.../example_configurations/AliExpress/CL-260/Configuration.h

Marlin/example_configurations/Anet/A6/Configuration.h

Marlin/example_configurations/Anet/A8/Configuration.h

Marlin/example_configurations/BQ/Hephestos/Configuration.h

Marlin/example_configurations/BQ/Hephestos_2/Configuration.h

Marlin/example_configurations/BQ/WITBOX/Configuration.h

Marlin/example_configurations/Cartesio/Configuration.h

Marlin/example_configurations/Creality/CR-10/Configuration.h

Marlin/example_configurations/Felix/Configuration.h

Marlin/example_configurations/Felix/DUAL/Configuration.h

...xample_configurations/Folger Tech/i3-2020/Configuration.h

...in/example_configurations/Geeetech/GT2560/Configuration.h

...e_configurations/Geeetech/I3_Pro_X-GT2560/Configuration.h

...n/example_configurations/Infitary/i3-M508/Configuration.h

Marlin/example_configurations/Malyan/M150/Configuration.h

...le_configurations/RepRapWorld/Megatronics/Configuration.h

Marlin/example_configurations/RigidBot/Configuration.h