I thought I might mention some alternatives. In the past I tried to use various software packages as level editors, and level editors are not always that different to UI editors.
Blender / Collada / Phyton
Great for advances meta data and 3D. IT also has 2D shape tools and bezier cubes that you can export to various 2d formats that you can parse in haxe. Your best bet would be the collada file format as it's just XML and can also describe non polygon objects like helpers, cameras,... Blender can be extended and customized with Python scripting.
Ink scape / SVG
Another open source one. Its main format is SVG which again is rather easy to parse. You can also use Adobe Animate to export svg's it's svg data is usually more clean than ink scapes or illustrator's.
Personal project where I used Adobe flash as a Risk level editor using SVG as the file format
Unity / C# Editor Scripts
Unity again is available as a free version as well. You could lay out or UI using their liquid 2D editor. Obviously for this workflow you might need to write and editor script (c#) to generate e.g. text assets for exporting the structure and meta data. I have done this a few years ago for a starling game for its levels since flash is not sophisticated enough for more complex meta data.
You can read more here:http://www.renderhjs.net/?doc=avengersGlobalChaos
Flash / Animate
But of course as many people have said, Adobe Animate is pretty good for vector layouts and containers with affine transformations. Adobe actually worked on their documentation for Editor scripts (JSFL) but that was still for Flash back then. Now with the Adobe Animate branding they move a bit more away from developers again.
You can find more information on how to add your own scripts, export formats or editor panels at:
I hope that many of those documentations are still valid as Adobe is not really doing a great support for Developers with their Adobe Animate package.