Qt Style Sheets support

The widget of this library supports the Qt Style Sheet properties of the Qt class they are derived from only where it makes sense. So you set the background-color of a PerceptualColor::MultiSpinBox. But you should not set it for a PerceptualColor::GradientSlider because the point of this widget is to always use the gradient as the background; the same applies for most widgets that showcase colors.

When using class names of this library as selectors in Qt Style Sheets, you have to substitute the namespace separator :: by -- to get a working selector: To select the class PerceptualColor::ColorDialog, use the selector PerceptualColor--ColorDialog.