LibDraw+LibGUI: Allow changing individual colors in a Palette

Palette is now a value wrapper around a NonnullRefPtr<PaletteImpl>.
A new function, set_color(ColorRole, Color) implements a simple
copy-on-write mechanism so that we're sharing the PaletteImpl in the
common case, but allowing you to create custom palettes if you like,
by getting a GWidget's palette, modifying it, and then assigning the
modified palette to the widget via GWidget::set_palette().

Use this to make PaintBrush show its palette colors once again.

Fixes #943.

Libraries/LibDraw/SystemTheme.h

@@ -39,6 +39,7 @@ enum class ColorRole {
 
     __Count,
 
+    Background = Window,
     DisabledText = ThreedShadow1,
 };