add javafx dependency

[bykka]

JavaFX is a standalone project, so add it to the dependencies otherwise it is not possible to compile these examples.

official documentation how to do it
https://openjfx.io/openjfx-docs/#maven

tested on
openjdk version "13.0.2" 2020-01-14
OpenJDK Runtime Environment (build 13.0.2+8)
OpenJDK 64-Bit Server VM (build 13.0.2+8, mixed mode, sharing)

[ruckustboom]

This is irrelevant until TornadoFX is updated to work with JavaFX 11+. The current version is for JavaFX 8. This is ongoing, and you can follow the progress in the unfortunately named jdk10 branch.

Hopefully these samples will be updated when TFX 2 eventually ships, but they haven't seem much love in a few years.