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.

Applications/SystemMonitor/NetworkStatisticsWidget.cpp

@@ -11,7 +11,6 @@ NetworkStatisticsWidget::NetworkStatisticsWidget(GWidget* parent)
         set_layout(make<GBoxLayout>(Orientation::Vertical));
         layout()->set_margins({ 4, 4, 4, 4 });
         set_fill_with_background_color(true);
-        set_background_color(SystemColor::Window);
 
         auto adapters_group_box = GGroupBox::construct("Adapters", this);
         adapters_group_box->set_layout(make<GBoxLayout>(Orientation::Vertical));