Use the covariant return type feature of C# 9 to allow method overrides to be more specific about their return type. By exposing this information, e.g. for Widget.Clone(), callers will have more information. This allows various casts to be removed as the information is now available within the type system.
- Add a new widget type for input and extend it from other input widgets
- Add a new label type that can be linked to an input widget
- Change the label color when the input's disabled state changes
- Add a property for arrows image collection (in drop-downs, scrollbars
and production tabs)
- Add a property for separators image collection (in drop-downs)
- Add hover and disable states to the drop-down separator
- Unify button, textfield and checkbox state suffixes
