Slider: add label substitution for certain values

Allows "Threads" option in advanced export setting to show "Auto" when set to 0

app/widget/slider/base/sliderbase.cpp

@@ -104,7 +104,17 @@ void SliderBase::changeEvent(QEvent *e)
 
 void SliderBase::UpdateLabel()
 {
-  label_->setText(tristate_ ? tr("---") : GetFormattedValueToString());
+  QString s;
+
+  if (tristate_) {
+    s = tr("---");
+  } else if (label_substitutions_.contains(GetValueInternal())) {
+    s = label_substitutions_.value(GetValueInternal());
+  } else {
+    s = GetFormattedValueToString();
+  }
+
+  label_->setText(s);
 }
 
 QVariant SliderBase::AdjustValue(const QVariant &value) const

app/widget/slider/base/sliderbase.h

@@ -49,6 +49,12 @@ public:
 
   QString GetFormattedValueToString(const QVariant& v) const;
 
+  void InsertLabelSubstitution(const QVariant &value, const QString &label)
+  {
+    label_substitutions_.insert(value, label);
+    UpdateLabel();
+  }
+
 public slots:
   void ShowEditor();
 
@@ -92,6 +98,8 @@ private:
 
   bool format_plural_;
 
+  QMap<QVariant, QString> label_substitutions_;
+
 private slots:
   void LineEditConfirmed();