How to build scalable GUI apps?
All the examples use global vars to maintain state across controls.
But this doesn't scale well or work well with large apps.
Is is possible to subclass? I've tried:
```nim
type
    CustomControl = ref object of Control
        state: int
```
and found that CustomControl isn't visible, whereas plain Control is.

(1) how do you subclass correctly -- or if this goes against the grain for NiGui, what is the correct approach?

(2) how do you communicate between controls or maintain state without globals?