LibGUI+LibGfx: Make scrollbar buttons a little bit thicker

The common thin-cap button look (1px highlight, 2px shadow) looks nice
on regular buttons, but the scrollbar didn't feel quite right.
This patch adds 1px of offset to the highlight, giving it a thick-cap
look (which I have named Gfx::ButtonStyle::ThickCap) :^)

Userland/Libraries/LibGfx/StylePainter.h

@@ -13,6 +13,7 @@ namespace Gfx {
 
 enum class ButtonStyle {
     Normal,
+    ThickCap,
     Coolbar,
     Tray,
 };