kcms/tablet: Add a warning before configuring virtual tablet devices

There's quite a few ways to get graphics tablet drivers. One of the most common is installing a user-space one, which usually comes with it's own bespoke configuration tool that will interfere with our KCM here.

I added a warning to the top of the KCM for said devices, which can be detected quite reliably. It's main purpose is to deter people from the KCM unless they really know what they're doing. As I said before, configuring the device in two places can really mess stuff up in non-obvious ways.