And this is how my first meaningful JavaFX GUI looked like:
Then I made the stage transparent and added a close image:
I got the idea from Zoran Pavlović blog. Thanks! :)
I'm already aware of several ways in which this application can be improved. Just in case you'd like to join me in learning JavaFX, I uploaded the code here.
An excellent tutorial to get you started with TortoiseHG versioning control system can be found here.