API for tablet pad button events

Many (Wacom) graphics tablets provide buttons (often referred to as "ExpressKeys"). Applications should be able to react to these buttons to do custom actions instead of relying on OS or driver-level functionality.

On Windows this seems to be possible using the Wintab Extended API: https://github.com/Wacom-Developer/wacom-device-kit-windows/tree/master/Wintab%20Tablet%20Controls

On Wayland this is possible with the https://wayland.app/protocols/tablet-v2#zwp_tablet_pad_v2 interface. On a basic level buttons have numbers, but some tablets also offer different button modes to allow assigning different actions per mode.

On a low level there could be a QTabletPadEvent class that is propagated to widgets.

On a higher level it would be nice to bind buttons to QActions, similar to keyboard shortcuts.