Fix KMainWindow saving incorrect widget settings

In certain cases KMainWindow::saveMainWindowSettings() could have been
called after mainwindow started destroying itself. Window settings would
be saved with incorrect child widget states. e.g. some widgets would be
saved as hidden even if they were visible before destroying.

Port of https://commits.kde.org/kxmlgui/d35a88289513c0420863b80aa6c1cb7d2c6e978f