Use a set of bitwise enum flags to set different keyboard accessibility
features and the necessary vfunc hooks in clutter input device so that
keyboard preferences can be passed to different input device
implementations.
The idea is to be able to configure either the clutter own
implementation of keyboard accessibility for evdev, or eventually
configure AccessX from the X11 clutter input device using the same
mechanism.
https://bugzilla.gnome.org/show_bug.cgi?id=788564
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.
