in blender you can easely create thousands of screens. 
i'd like to have every screens from bpy.data.screens that are not visible deleted, how can i do that ? 

i know that 

```
for screen in bpy.data.screens:
    bpy.ops.screen.delete({'screen': screen})

```

delete every single screens, that's a goo thing, but i don't know how to detect the visible/invisible ones ?