GUI: Modern "non-fancy" themes do not take shading into account when building layout

[mikrosk]

When showing the launcher in the compact (so called "not fancy") mode, the list widget overflows into the buttons when drawing.

Most of the time this is not noticeable but I have found a nice repro:

• hover over a button (i.e. "Add game")
• press key down
• notice how part of the button is redrawn from the background layer (the same layer in which the list widget's background is)

Reproduced on latest master (ddfa47893b21e74c4dcdd474f3ac0f44a4466a05 at the time being), OpenGL backend, window 572x364 (see the attached picture for demonstration).

Not reproducible on the built-in theme (as it has no shadows).