xcb: use keyboard event source window to determine auto-repeat value

The xcb auto-repeat checking code checks for queued events on the target
(focused) window. This breaks down if you have a native child widget,
and QKeyEvent::autoRepeat is never true.

Task-number: QTBUG-21500
Change-Id: Ia979edfa8a3afce07a3e1cbaa778541ffb9ce5dc
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>