Add a button to settings allowing to validate the corner points

[j-be]

The plugin cannot determine if a given probe point is reachable or not. Afaik. It would need to take into account at least the following from Marlin:

• X_MAX_POS and Y_MAX_POS
• NOZZLE_TO_PROBE_OFFSET - can be queried using M851 - XYZ Probe Offset
• PROBING_MARGIN

but it seems it cannot get either of those.

So maybe we just add a button which makes one round like:

1. Click button
2. Send G30 <corner>
3. Dialog to user "Does printer move? Yes - No"
4. Try to correct corner point, goto 2

EDIT: Added reference to M851