kcms/tablet: Unify tablet pen and pad selection into one combo box

In the KCM's current state, the device selection is a bit wonky. This is because you can select two different "logical devices" and configure each. These are, of course tablet pens and tablet pads.

Now, these are unified into one combo box and you select each logical group - if they can be grouped. If you have a stylus pen without a pad, or a tablet pad without a stylus then the KCM will function like before.