Revise string literal usage

This commit covers src/gui folder.
PR #16757.

src/gui/previewlistdelegate.cpp

@@ -57,7 +57,7 @@ void PreviewListDelegate::paint(QPainter *painter, const QStyleOptionViewItem &o
         {
             const qreal progress = (index.data().toReal() * 100);
             const QString text = (progress >= 100)
-                ? QString::fromLatin1("100%")
+                ? u"100%"_qs
                 : (Utils::String::fromDouble(progress, 1) + u'%');
 
             m_progressBarPainter.paint(painter, option, text, static_cast<int>(progress));