Remove FHeaderAction & replace it with FButton.icon() (context-aware styling)

[Pante]

Is your feature request related to a problem? Please describe.
FHeaderAction is almost identical to FButton.icon(). It would be great if there was a way to simplify the public API by removing FHeaderAction and allow users to just use FButton.icon() instead. However, the default style for header actions is different from buttons. We'll end up with FHeader's icon style -> FButton.icon() -> FButton's icon style, therefore making it impossible to override the icon style with our current system.

We need "context-aware styling". However, it remains unknown if the benefits of it outweigh the cons. Specifically, the results of placing widgets under certain parents might prove to be counter-intuitive. Does that outweigh the benefit of creating wrapper types just to provide a different default style?