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