Allow sub-buttons inside the buttons in a horizontal stack

[SLaks]

I'd love to add sub buttons inside the popup-opening buttosn in a horizontal stack.

Example uses:

• A button that opens a popup with every light in the second floor, with a sub button to turn them all off without opening the popup
• A button that opens https://github.com/PiotrMachowski/lovelace-xiaomi-vacuum-map-card, with a sub button or 2 to immediately run the vacuum.

Relatedly, I'd love to have separators or spacers between buttons in the horizontal stack (I can probably fake that with card_mod)