As a step towards making this a real game, I need to be able to draw sprites and click buttons.
This functionality is basically available in the legacy code, so it was just a matter of exposing it to the new custom language the game is written in.
For the GuiButtons, they did exist in legacy code, but I decided to rewrite them from scratch. But it was only a small ammount of work, as a button is a composite of two images (unpressed and pressed) and a text label on top. To handle clicks the Gui element can just be flagged as clickable, and the engine handles the rest.