So. There's a little bit of progress on the editor.
I can now load and save Json-files. I can add and remove rooms (views) from the grid, or just swap the background image of a view. I can navigate the grid with the keyboard arrows (scroll), or by typing in new view coordinates and clicking the 'goto' button.
One thing did give me some trouble (and here you can really see how inexperienced I am), and that is saving the modified Json-file to the correct directory.
For some reason, I assumed that all file-related methods (eg. Assets.exists('json/file1.json') or File.getContent('json/file1.json')) referred to the root of my project.
Of course, that is incorrect since Haxe is a compiled language. So every path is relative to the root of the compiled project !
It took me a little while to figure out how to prevent saved files from ending up in the 'bin' directory ...
But anyway. Lesson learned.
The next step is to be able to select a view and add navigation info to it.