LibGUI+LibDraw: Add "Palette" concept for scoped color theming

GApplication now has a palette. This palette contains all the system
theme colors by default, and is inherited by a new top-level GWidget.
New child widgets inherit their parents palette.

It is possible to override the GApplication palette, and the palette
of any GWidget.

The Palette object contains a bunch of colors, each corresponding to
a ColorRole. Each role has a convenience getter as well.

Each GWidget now has a background_role() and foreground_role(), which
are then looked up in their current palette when painting. This means
that you no longer alter the background color of a widget by setting
it directly, rather you alter either its background role, or the
widget's palette.

Libraries/LibDraw/Color.h

@@ -4,6 +4,7 @@
 #include <AK/String.h>
 #include <AK/Types.h>
 
+enum class ColorRole;
 typedef u32 RGBA32;
 
 inline constexpr u32 make_rgb(u8 r, u8 g, u8 b)
@@ -11,38 +12,6 @@ inline constexpr u32 make_rgb(u8 r, u8 g, u8 b)
     return ((r << 16) | (g << 8) | b);
 }
 
-enum class SystemColor {
-    DesktopBackground,
-    ActiveWindowBorder1,
-    ActiveWindowBorder2,
-    ActiveWindowTitle,
-    InactiveWindowBorder1,
-    InactiveWindowBorder2,
-    InactiveWindowTitle,
-    MovingWindowBorder1,
-    MovingWindowBorder2,
-    MovingWindowTitle,
-    HighlightWindowBorder1,
-    HighlightWindowBorder2,
-    HighlightWindowTitle,
-    MenuStripe,
-    MenuBase,
-    MenuSelection,
-    Window,
-    WindowText,
-    Button,
-    ButtonText,
-    Base,
-    ThreedHighlight,
-    ThreedShadow1,
-    ThreedShadow2,
-    HoverHighlight,
-    Selection,
-    SelectionText,
-
-    DisabledText = ThreedShadow1,
-};
-
 class Color {
 public:
     enum NamedColor {
@@ -71,7 +40,6 @@ public:
 
     Color() {}
     Color(NamedColor);
-    Color(SystemColor);
     Color(u8 r, u8 g, u8 b)
         : m_value(0xff000000 | (r << 16) | (g << 8) | b)
     {