Themed window title bars

[_Hajo_]

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.

[Yona-TYT]

Wow this is amazing!, I want to see that implemented already.

[_Hajo_]

Demo builds,
https://github.com/Varkalandar/simutrans/releases/tag/Nightly

Code (ugly),
https://github.com/Varkalandar/simutrans/tree/ui_improvements