Fix crash in KSelectAction
If the destroyed() signal from QComboBox, which is connected to the comboBoxDeleted() slot, is emitted while the KSelectAction object is being taken down, there is a crash is QList::removeAll(). Fix the issue by disconnecting the signal in the KSelectActionPrivate() destructor.
This fixes a crash when closing the KMail composer window, reported by mlaurent.
Add a comment explaining why the KSelectActionPrivate() destructor is virtual, the comment is taken from an old commit by pino in kdelibs circa 2007 :)