Various keyboard accessibility fixes

Most important fix here is the "enable" setting, we still have this wrong even after commit 5c27bf6a and commit 34ee4602 ...

The keyboard accessibility setting "enable" is actually even more misleading that initially anticipated, as it does not control the entire keyboard accessibility feature, but just the "enable by keyboard" feature, i.e. being able to enable or disable stickykeys or slowkeys using various keyboard actions.

Yet the accessibility features should still work even if the "enable" setting is unset, those can be controlled by the accessibility menu in GNOME Shell for example.

Change the clutter/evdev implementation to match that behavior as found in the x11 backend, so both backends are now consistent.