Allow modification of default graphic decoration

[effad]

The default graphic decoration cannot be modified in any way (e.g. fontsize) at the moment. This should be made possible by:

• Changing the return type of net.synedra.validatorfx.DefaultDecoration.createGraphicDecoration(ValidationMessage) to GraphicDecoration
  • createStyleClassDecoration should be changed as well for consistency
• Adding a getDecorationNode method to GraphicDecoration, which will allow access to the decoration node.