kstyle: Use a QTimer to defer mutating windows until after painting is complete
By using a QTimer to defer mutating windows w/ debouncing behaviour, we can avoid causing recursive paints which lead to crashes. 20ms is an arbitrary value, but it seems to be common in Qt for 'we need to defer this until this process driven by the event loop is completed' stuff, so we copy that here.
BUG:432246 FIXED-IN: 5.21