clutter-main: Add hooks to plug kbd accessibility

On X11, when AccessX is enabled, all X11 clients benefit from the
AccessX features, including gnome-shell/mutter, meaning that special
keys like the overview or other shortcuts are also affected.

To achieve the same in Wayland, we need to implement the same features
in clutter main rather than the Wayland backend, so that all depending
code within the compositor but also Wayland clients (which rely on the
compositor to get keyboard events) similarly benefit from the
accessibility features.

Add hooks to the clutter main loop to be able to implement such
features.

https://bugzilla.gnome.org/show_bug.cgi?id=788564