The idea to sketching UI, or other types of drawing, using text file is not new. Text can be embedded into text files so a whole document can be defined using only text. This is the approach used by Sphinx, with it you can use reStructuredText to define your HTML or PDF documentation.

Sphinx is integrated with PlantUML so you are able also to define UI, UML diagrams, charts using text.

The subproject used by PlantUML to define UI sketch is called Salt. What I do not like about Salt it's that it uses lines, curves and text to draw a UI sketch. This way the result is not realistic.
And ok... I would like to exercise with ANTLR.