Adds a ComponentWithBody abstraction that is now used by button and window (might be later used by layouts as well)

The button abstraction was always a bit off, in my opinion. I know that it's common in immediate UIs to have buttons inside if conditions, but I find this problematic, as one needs to be very careful with the use of else if clauses (buttons inside an else if won't render when the previous buttons are clicked), which in turn makes the code more statement oriented.

While this is an imperative library, I think it's still valuable if everything is an expression, so I hope this simplifies things.