kcms/tablet: Use a QML MouseArea to accept tablet events in the tester
We were using the same method in other parts of the KCM to monitor tablet events. This is only really useful passively, as it does not intercept said events and pass them right along.
This has an accidental effect, as is the case with the new tablet tester tool. In Qt 6.8, tablet tools can now drag windows which hid a bug here. Since we passively monitored tablet events, it will send regular tablet events through and onto the window the tester sits on. This makes it drag while trying to draw.
A simple way to fix this is by putting an active input handler in the drawing area to catch said events.
Edited by Joshua Goins