kcms/tablet: Add tablet tester

Joshua Goinsrequested to merge
work/redstrate/the-tablet-tester into master

This adds a simple tablet tester to the KCM, modeled after a certain KDE application's own:

Screenshot_20240114_160710

Currently this only reports the X,Y of stylus events. Eventually it can be expanded with tilt and button information in the future.

Needs !1976 (merged)

Edited by Joshua Goins

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