Add icon theming

[Tc-Fast]

align with
modelica/fmi-standard#2061

[pmai]

Design Meeting 2025-10-20: In order to provide this as a backward-compatible extension, a layered standard would be the appropriate way to handle the need for additional element content in the XML (e.g. via annotations), that can still be backward compatible with SSP 2.0.

[DagBruck]

To be frank, I think this goes beyond what is meaningful for core SSP. To avoid the problem people can design icons that work in both "light" or "dark" mode or whatever lies in-between. Or tool vendors can join forces to provide a coherent experience.

I suggest that those interested in this feature can start to agree on annotations, and if that plays out well move on to a layered standard.

[Tc-Fast]

We also discussed this issue in the FMI Meeting.
A FMI/SSP cross layered standard, incorporating theming, localization, widgets etc. may be the best solution.