KWin used to do a quirky trick to send key events to the topmost QQuickItem
rather than things going to the activeFocus item.
Sending it to the window previously would have failed as the window
didn't think it was active.
Since 66986d4afddcd09c28fe3addb0caa09279eda10f we can just let the
window process the events in a normal QtQuick manner.
It will require tabboxes to set focus correctly.
The ones I tested did.
Most analysis for this patch was done by Chris Holland.