Add option to toggle hierarchical rendering mode for header icons

[waydabber]

With hierarchical rendering off:

[waydabber]

[KrzysztofPrzygoda]

This new hierarchy is way better than the current one:

Although, native switch would be the best, I think. Example from Systems Settings:

[waydabber]

Yes, I was thinking about that as well. It's just that I wanted the button to reflect whatever it is doing (for example if the button is set to toggle the backlight, it has a bulb icon in it). Will still think about it, but I think the current implementation is generally ok.