clutter: add hooks for kbd a11y configuration

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