Now that we have private, per-event platform data, we can start putting
it to good use. The first, most simple use is to store the key group
given the event's modifiers. Since we assume a modern X11, we use XKB
to retrieve it, or we simply fall back to 0 by default.
The data is exposed as a ClutterX11-specific function, within the
sanctioned clutter_x11_* namespace.
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