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.

DevTools/HackStudio/Editor.cpp

@@ -68,7 +68,7 @@ void Editor::paint_event(GPaintEvent& event)
             rect.set_width(rect.width() - vertical_scrollbar().width());
         if (horizontal_scrollbar().is_visible())
             rect.set_height(rect.height() - horizontal_scrollbar().height());
-        painter.draw_rect(rect, SystemColor::Selection);
+        painter.draw_rect(rect, palette().selection());
     }
 }