Code is not presentable yet, but I have something that works and is compatible with old themes. So it won't break anything existing. Old themes just show the usual old style window titles.
Wow this is amazing!, I want to see that implemented already. 8)
Demo builds,
https://github.com/Varkalandar/simutrans/releases/tag/Nightly
Code (ugly),
https://github.com/Varkalandar/simutrans/tree/ui_improvements