Allow configuration of the AES stylus "eraser button" (e.g. to right- or middle-click)
AES pens are newly supported by libwacom and use an "eraser button" rather than a dedicated tool at the opposite end of the pen. When this button is held down, the driver sends the stylus tool out of proximity and starts sending events from the eraser tool instead. This default behavior allows you to have access to an eraser within applications like GIMP but limits you to only having one programmable button. Having the ability to override this behavior in g-c-c would be handy for users who would get more use out of a second configurable button (which could e.g. right- or middle-click) than they do out of an eraser.
This is probably a pretty complex feature to implement since it would rely on monitoring the eraser device for proximity events and then sending a synthetic button event in response. I know GNOME has the infrastructure in place to do this in response to button events (which is how current stylus and ExpressKey configuration works), but I don't think it has anything similar for proximity...
In addition, it would require some modifications to the UI being designed in #1017 to ensure that the eraser button points to a dropdown menu. We also need to figure out how we want the pressure curve control to also point to the same button (just a bifurcated leader line?)