kcms/tablet: Add configurable pen pressure curve
This adds a new feature to the Tablet KCM, allowing artists to tweak the pen pressure curve of their pen (or eraser.) It's exposed in the KCM as a curve, allowing up to two fully-controllable points of articulation. There are two partially-fixed points for adjusting the bottom and top thresholds in the case where the pen needs those.
Needs kwin!4920 for configuring the pen pressure on the KWin side.
Edited by Joshua Goins