Add support for switching scroll methods

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Add support for changing the button-scrolling button

This currently exposes the libinput button name, which isn't ideal. Needs to
be switched to X button numbers.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Use the button conversion helper for normal button events too

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>