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.
7b2dd7e116