Can we have a convenience to mutate class by adding a class name instead of replacing the whole attribute value?

[Frizlab]

Currently I don’t think there’s a way to modify an attribute by adding stuff to the value; we have to replace everything.

Example of API use I think of, and why it is needed:

Division{ }
   .class("full-height")
   .modify(if: isToday(), element: { $0.addClass("today") /* <== HERE */ })

Possibly some other attributes would benefit from this.

More generally we could have the possibility to access the value of an attribute to be able to at least do something like element.class(element.class + " today") (a bit less elegant, but clients could add an extension on Division in which they would implement addClass that would do the addition; currently AFAICT doing the extension is not even possible).

[mattesmohr]

I think, I am on board with the idea and wonder why this didn't come up earlier. I will have a look into this.