RoundButton: Rework from the ground up
It has a somewhat Breeze-influenced appearance, and probably won't blend too well with other styles, because we don't have QStyle counterpart which we could delegate painting to.
On this video showcase, the button in the bottom left corner has regular focus policy and is keyboard-navigable. The one on the right is stretched vertically and has an extra right padding — notice how inset (background's own padding) is automatically adjusted to maintain cantered position relative to the icon. The idea behind the math trick was taken from PlasmaComponents implementation, but adjusted (CC @mart I believe that formula was slightly off, so I took the liberty of swapping pieces left and right).
CC @teams/vdg