I used SceneBuilder to generate the GUI for my app, and it auto generated a FXML file with all that data. How would I go about representing it in a UML class diagram, if at all?
You simply represent it as artifact (p. 654 of UML 2.5):
19.3 Artifacts
19.3.1 Summary
An Artifact represents some (usually reifiable) item of information that is used or produced by a software development process or by operation of a system. Examples of Artifacts include model files, source files, scripts, executable files, database tables, development deliverables, word-processing documents, and mail messages.
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19.3.4 Notation
An Artifact is presented using an ordinary Class rectangle with the keyword «artifact». Alternatively, it may be depicted by an icon (such as the document icon shown in Figure 19.9). Optionally, the underlining of the name of an artifact instance may be omitted, as the context is assumed to be known to users.
A Manifestation is notated in the same way as an Abstraction, that is, as a dashed line with an open arrow-head, labeled with the keyword «manifest».