[FXMLLoader]: Null builderFactory makes it impossible to load many types of classes

Setting builderfactory to null makes impossible to load anything that needs a builder (has no default constructor) or that uses a special builder (Scene, Font, Image, etc...).

I think we should separate the concept of user-provided builder and default builder, so that the default builder is used if no user-provided builder for the class was created from provided builder factory (if any). This allows user to specify it's own builders without delegating to the JavaFXBuilderFactory explicitly.