This function call only applies to tablets, and thus will error
out unless it's called with CLUTTER_TABLET_DEVICEs. This will
allow setting absolute/relative mapping on those on the fly, as
this is optional.
Those map closely what we get from libinput. Button events have
been made its own separate struct, its semantics fall somewhere
in between of ClutterButtonEvent and ClutterKeyEvent, so is better
emitted as its own set.
