Port deprecated QList::toSet(), QDesktopWidget::screenGeometry()

apps/kmix.cpp: In member function 'void KMixWindow::loadBaseConfig()':
apps/kmix.cpp:498: warning: 'QSet<T> QList<T>::toSet() const [with T = QString]' is deprecated, use 'QSet<T>(list.begin(), list.end()) instead.'
gui/dialogstatesaver.cpp:104: warning: 'const QRect QDesktopWidget::screenGeometry(int) const' is deprecated, use 'QGuiApplication::screens()'
/usr/include/qt5/QtWidgets/qdesktopwidget.h:79: note: declared here
gui/dialogstatesaver.cpp: In static member function 'static void DialogStateSaver::saveWindowState(QWidget*, KConfigGroup&)':
gui/dialogstatesaver.cpp:142: warning: 'const QRect QDesktopWidget::screenGeometry(int) const' is deprecated, use 'QGuiApplication::screens()'
/usr/include/qt5/QtWidgets/qdesktopwidget.h:79: note: declared here