[Class editor] Update endpoint for classes does not support the new icons

[vin0401]

At the moment the current update endpoint for class definitions only accepts simple strings for the icon.
Instead we should support the common icon syntax.

[lukmzig]

@vin0401 we can do this only for the general settings o class definitions - here we set the value directly so we can transform it from "new icon syntax" to a string. The issue is that the core models work only with the string currently.

The bigger problem are layouts. These values are directly passed and retrieved from core and they work only with the string.

Without going into too much details, if we really want to make this consistent we should change it for all class definitions, object bricks, field collections, layouts etc. .. we cannot do this cleanly without some "hacks" without core model changes.