The code that 'understands' <properties><backgroundColor>...</> should be removed

See attached FXML file.

The background color for the preview is now saved as a PI. However for convenience/compatibilty reasons we still understand the 'old' format - which used to save the background color in the properties map of the root node. This made sense since the first public ea/beta versions of SceneBuilder used to understand this property.

Post 1.0 we should stop looking in the properties map for a background color.