Fix window border corner appearance

[git-f0x]

This removes the background of the header bar and content containers, and moves it to the container wrapping both, in order for the border to be drawn properly.

The second commit matches the spacing of buttons in header bar sections to the designs.

Edit: Apps without content containers but with a header bar won't benefit as much (they do see a slight improvement), but not sure if it's possible to do something about that.
A thing that would enable fixing this (and would be useful for other things), is for the border/border color to be settable per-side, which would enable setting the border in the individual containers, rather than in the wrapping container in app/mod.rs, which could then likely be removed (though not sure about the drawing logic in the corners).
An API for that could be just to have the border color as a Vec, with similar convenience features as e.g. padding (color: Color would be equivalent to color: [Color, Color, Color, Color]).

Should I restrict the value of the radius_s - 1.0 values to have a minimum value of 0 or radius_0, so that if someone sets the radius to 0, it doesn't cause weird visual issues?