XCB: Async operations between XCB and X11 cause flakiness in QXcbScreen::topLevelAt()

The Qt XCB implementation knows about a window, when it has been mapped.
QApplication::topLevelAt() eventually lands in QXcbScreen::topLevelAt(). This method iterates from the screen's root window to the topmost child via xcb_translate_coordinates.
This call requests coordinate translation from the X11 server, which runs in a separate thread. It might not have been fully updated, when receiving the call - even though the window is mapped and has an ID.

That leads to flakiness, e.g. in tst_gestures.