LibGfx+Base: Add Tray/TrayText theme colors and Gfx::ButtonStyle::Tray

A "Tray" is a sunken container area for widgets. The first intended
client of this style is the GUI::FilePicker's common locations frame.

Thanks to @nvella for coming up with the term "Tray" :^)

Userland/Libraries/LibGfx/SystemTheme.h

@@ -99,6 +99,8 @@ namespace Gfx {
     C(ThreedShadow2)               \
     C(Tooltip)                     \
     C(TooltipText)                 \
+    C(Tray)                        \
+    C(TrayText)                    \
     C(VisitedLink)                 \
     C(Window)                      \
     C(WindowText)