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.
Return StylePainter::paint_button() before color initializations
when possible and replace an overdrawn rect in paint_button_new()
with more explicit shadow lines. Also standardize some comments.
This adds a "shine" effect to the bottom-right edges of pressed
and checked buttons, complementing the sunken shadow already
present at top-left and creating greater illusion of depth.
It is similar to the effect used in classic Windows and is
theme agnostic in Serenity, but produces the best result when
ThreedHighlight is a radiant color of Button.
