LibWeb: Separate layout tree rendering into phases

CSS defines a very specific paint order. This patch starts steering us
towards respecting that by introducing the PaintPhase enum with values:

- Background
- Border
- Foreground
- Overlay (internal overlays used by inspector)

Basically, to get the right visual result, we have to render the page
multiple times, going one phase at a time.

Libraries/LibWeb/Layout/LayoutWidget.cpp

@@ -65,9 +65,4 @@ void LayoutWidget::update_widget()
     widget().move_to(adjusted_widget_position);
 }
 
-void LayoutWidget::render(RenderingContext& context)
-{
-    LayoutReplaced::render(context);
-}
-
 }