Fix a few deprecation warnings in Qt 5.14.1

Fix a few deprecation warnings in Qt 5.14.1

These changes fix a few instances of the following type of warning:

  error: ‘void QListWidget::setItemSelected(const QListWidgetItem*, bool)’ is deprecated: Use QListWidgetItem::setSelected() instead [-Werror=deprecated-declarations]

as well as similar warnings for setItemHidden() and
setItemExpanded(). These are just warnings now, but it is planned to
remove the methods for Qt 6:

  https://bugreports.qt.io/browse/QTBUG-73048

I tested that LyX can still be built against Qt 4.8.7 with this
commit. Indeed, these methods have been deprecated for a while (it
is just that QT_DEPRECATED_WARNINGS was only turned on by default
starting with 5.13.0). See, e.g.,

  https://doc.qt.io/archives/qt-4.7/qlistwidget-obsolete.html