kcms/tablet: Add pen calibration tool
This adds a new pen calibration window to the Tablet KCM, if supported by the pen device. This allows to fix possible parallax issues with the out-of-box calibration matrix.
It builds off of the existing calibration matrix feature already present in KWin (which then feeds into libinput). Is basically a souped up version of https://github.com/kreijack/xlibinput_calibrator. As shown in the screenshot, you press four targets which exist in fullscreen quadrants. You can reset the calibration back to default as well, if you mess up. Turn up your sound, there's even sound effects
I've been testing this on my two pen tablets on hand, and it seems to be doing what it should. More testing is very welcome though, I'm still learning this maths.
Needs !1976 (merged)