Regression: Serious Focus Problems With X11

This is like QTBUG-51573 but caused differently. With X11 and under Linux, when a menu is closed by clicking outside its window, the child widgets of that window remain unresponsive (no response to a hovering cursor) until the window is clicked. The same thing happens after resizing a window from its size grip. There are other cases too. None of these happened with Qt-5.7.

The problem should have been caused by the latest changes in `QXcbWindow::handleXIMouseEvent()` because after I reversed the changes to Qt-5.7.1, the above-mentioned issues disappeared completely.