libkwindevices: Reset calibration matrix to default in defaults()
Previously when pressing the "Defaults" button in the Tablet KCM, the calibration matrix is not reset. The only way to do so was going through the calibration process again. Turns out, if you messed up your calibration badly it was impossible to reset it. This is because the calibration can only finish by using the tablet pen you can't use. Oops.
Now the "Defaults" button in the KCM resets the calibration matrix, which is accessible by mouse or touch.