feat: Add JRuby controller skeleton generation

[byteit101]

Adds JRuby as a language to the controller skeleton dialog. Note that JRuby doesn't need the fields, as JRubyFX inserts all of them automatically, this is principally for the methods/callbacks, and field reference.

Based on discussion #595

Progress

• Change must not contain extraneous whitespace
• License header year is updated, if required
• I don't understand this: The PR name must follow the pre-defined format
• Verify the contributor has signed Gluon Individual Contributor License Agreement (CLA)

[Oliver-Loeffler]

There exists a naming convention for PRs requiring prefix to make clear what their purpose is.
https://github.com/gluonhq/scenebuilder/blob/master/CONTRIBUTING.md#pull-request-standards
In this case I would consider: "feat: add functionality to generate JRuby controller skeleton" as a suitable name.

[abhinayagarwal]

@Oliver-Loeffler Can you review this PR again? Author has addressed your comments.

[Oliver-Loeffler]

License headers look okay now. Also I've tested this PR and it works fine. Also new languages can be added by just implementing the SkeletonConverter.

[Oliver-Loeffler]

Just closed the review.