Add icon theming

[Tc-Fast]

Currently the FMI standard allows to add a single kind of icon. Which is usually assuming a "light" themed editor.
I would like to add a second icon for "dark" themed editors.

[chrbertsch]

Pierre: in the SSP project the proposal was to have a layered standard .... it could be a common layered standard enhancing also other aspects for graphical improvements
Nikolai: e.g. also localization. Or widgets. We should discuss this further.