GUI::TabWidget has long has a TabPosition::Bottom option, but we still
rendered the tab buttons the same as TabPosition::Top.
This patch implements a custom look for bottom-side tabs. I've done my
best to match the look of the top-side ones, but there might be some
improvements we can make here. :^)
StylePainter's behavior is now handled by a static instance
of BaseStylePainter. The original static behavior of StylePainter
is left as-is for API compatibility.
