message: Add guard around Slider ValueChangeSignal
  This prevents infinite recursion in case the handler method tries to
  unconditionally change the Slider's value.
diff: @@ -304,9 +304,13 @@ void Slider::draw() const
  void Slider::value(ValueType newValue)
  {
  + const auto oldValue = mValue;
  mValue = std::clamp<ValueType>(newValue, 0, mMax);
  + if (mValue != oldValue)
  + {
  mSignal(mValue);
  }
  +}
  Slider::ValueType Slider::value() const
  