img: apply padding around window icon only horizontally

img: apply padding around window icon only horizontally

16dbdc64 changed the padding around the app icon in the titlebar to be
applied both vertically and horizontally rather than only horizontally
because it was more natural from a developer's perspective, but some users
complained about the smaller icons in certain configurations.

So let's undo the change in 16dbdc64 and apply the icon padding only
horizontally for now.

We can add configurations for the icon padding (or icon size independent
from window.button.{width,height}?) later.