kstyle: support translucent tools areas

Any colour in a KColorScheme can become translucent, or even 100% transparent. This holds true for the Header colour set used to draw the background of the unified window titlebar/toolbar/menubar area. The decoration supports translucent backgrounds, but the tools area does not; the window remains fully opaque.

This remedies that discrepancy by making the window able to draw translucent backgrounds.