Add button to execute an M48 - Probe Accuracy Test and use it to validate accuracy

[j-be]

Currently, it is hardly possible to determine whether Accuracy threshold in settings has a value matching the individual printer.

Using an M48 we get a report like:

Mean: 0.005000 Min: 0.003 Max: 0.010 Range: 0.007
Standard Deviation: 0.002500

Should be Accuracy threshold according to Range. I think half of it is the hard limit, so anything:

• Accuracy threshold >= Range: All good
• Accuracy threshold between Range and Range / 2: Warning - that's ambitious
• Accuracy threshold <= Range / 2: Error: no way that's gonna get you anywhere

[MirekStanek]

This is a really great idea. I would love to see that in action but I had Z_MIN_PROBE_REPEATABILITY_TEST turned off by default. So you might need a way to find out if it's on/off in config.