[JavaFX Tutorial] Further elaboration on problem 1 in part 4

[gongg21]

Part 4 of the JavaFX tutorial first introduces the problems of implementing JavaFX in just Java alone. Problem 1 can be elaborated on, providing evidence to allow readers to understand why it is a problem more clearly:

The Main class attempts to do it all. Code for visual tweaks, listeners and even utility methods are all in one file. This makes it difficult to find and make changes to existing code.

That is, perhaps we could add the Main class code that was implemented by Part 4 in the tutorial into a dropdown. This way, students can expand the panel and see the code, with further explanations.