Add "Link", "ActiveLink" and "VisitedLink" colors to the system theme
definition, and implement support for them in LibHTML.
Note that <body link="foo" alink="bar" vlink="baz"> takes precedence
over the system colors. Author style also takes precedence, since we
only fetch the system color in case the CSS color is -libhtml-link.
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.
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.
