Probe on bed where you want and less configurations

• CAUTION: The X and Y position configurations are now merged to a combined X,Y parameter,
  Please update your Configuration!
• The CALIBRATE_Z command has an optional parameter for the probing position on the bed
• The restriction for a relative reference index in the "bed_mesh" section is removed
• Some other small changes from the failed Klipper PR are merged back now
• Example configurations are removed because they were just examples and confused many people