Visual refinement for nested rounded corners

[git-f0x]

The nested rounded corners that appear in some places like the window and window sidebar corners currently use the same radius variable. Since there's a padding of 8 between them, this leads to the outer radius (window corner) visually sticking out:

It could be improved by increasing the outer radius by 1/2 of padding (adding the entire padding is mathematically correct for achieving concentric corners, but doesn't look good), which visually aligns the corners:

This would be implemented by having the outer radius use the following rule:
window_corner_radius = radius_s().map(|radius| if radius < 4.0 { radius } else { radius + 4.0 })
This also ensures that the Square style remains square, and is unaffected by this change.

This issue is created to gather feedback on the change.
CC @pop-os/ux